I Live in Philadelphia but work the NYC, Philadelphia, Baltimore and Washington DC Markets. Is signing with 1 Agent good enough to get work in all of those markets or do I need different Agents in each Market. How many Agents can I sign with in each Market.

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1st Let me make sure you understand the Process - There is a lot of information here but you need to review ALL of it before you start signing multiple Agents. The information is based on your being NON-UNION - Being Union some of this will not apply but some of it still will apply.
Your Agent(s) are your Employee(s) Hired by you, to find you work and negotiate the best possible deal for you, and paid by you in the form of the commission They earn from the Paid work they get for you.

Yes, Agents have to accept you as a Client and they want to be associated with the Best Actors - In some markets it is Difficult to get an Agent But you are still the boss - Unless you sign a Contract that Limits your Rights, You can fire the Agent at any time IF they do not do their job and find you good paying work.
There are many reasons to NEVER- EVER-EVER sign a contract the day someone hands it too you, and Hundreds of Reasons to RUN, RUN, RUN if ANYONE tries to talk you into signing immediately. You ALWAYS state you need to read it over at home and your LAWYER needs to review it as well. (Even if you do not have a Lawyer - you tell them you do)
Everything is in that Contract and you are bound by its terms.
Things like how you can cancel the contract are in there - Written notice??, written notice 30 days in advance of cancelation, 60 days in advance, 90, 120, 180Its whatever they wrote in the contract - Simply because you agreed to it.
What happens after you cancel the contract? Do you still owe them a commission on future work you do? For how long? 1 year, 5 years 10 years If the contract says you do - then yes for however long you agreed to pay - Simply because you agreed to it.
When does the Contract expire? Most are 3 years, some are 5 years but some NEVER EXPIRE because they have a clause stating that the contract AUTOMATICALLY RENEWS FOR ANOTHER 3/5 Years UNLESS CANCILED. So if you agreed to a 180 Day written advance cancellation notice, You will sit around for 6 months after you notify the Agent you are not renewing the contract- waiting for the contract to expire - Simply because you agreed to it.
What is the rate of Commission Charged? is it 10%, 12 1/2 %, 15%, 17 1/2%, 20%, 25% And on what type of work - There are Contracts that are 10% for feature films, 15% for Commercials, Industrials and 25% for Print

The Biggest Problem in the Contracts and 1 that you really NEED to understand is "The Exclusive Clause" in that contract because you are obligating yourself to pay the Agent a commission on all the work covered in the Exclusive, no matter who got you the work or if you already paid the person that got you the job a commission.
The Baltimore and Washington DC Markets and many other parts of the country are FREELANCE markets. All Casting Directors Keep their own files and Hire mostly from them - Seldom do they go to the Agents for additional Talent and If they do it is not the Majority of those needed. Most Agents do not have a contract here and Most of those that do are Non-Exclusive - as with everything - there is always an Exception to anything.
Philadelphia is more like Baltimore and Washington DC But Casting Directors go to the Agents More often and for a larger % of the Talent needed.
New York (and LA ) is TOTALLY DIFFERENT
In NYC (and LA ) CASTING DIRECTORS almost never handle PRINCIPAL ROLES and EXTRAS like here in the Baltimore / Washington DC Markets. Those Hiring EXTRAS keep their own files and Hire Exclusively from those files - NO AGENTS INVOLVED.
.Those Casting for Principal Roles rely almost exclusively on the Agents for their talent.
Most Casting Directors SPECIALIZE in part of the market because the market is large enough for their offices to be profitable while SPECIALIZING in 1 or 2 segments of the market. A Casting Director may do Feature Films, another Might do TV (or Cable) Shows and Made for TV (or Cable) Movies, while still 3rd Casting Director might do Commercials, Industrials and Print.

Many Agents Develop strong working contacts along those same lines - feeding their talent to multiple Casting Directors that do Feature Films or TV Shows.
You have to know what any Agent does BEFORE you negotiate a Contract with them, Yes, EVERY part of EVERY contract is NEGOTIABLE and ANYONES refusal to NEGOTIATE is the other persons queue to LEAVE.
Back to the Exclusive Clause
The Exclusive Clause in any contract is what You and the other party agree it is, and YOU HAVE LEGALLY AGREED TO BE BOUND BY ITS TERMS.
The Exclusive Clause covers 2 main parts of your FUTURE WORK, The Type of work you do, and the Location that you do your work in.
In the Exclusive, Tight narrow definitions benefit you, Broader Definitions benefit the Agent.
An Example - lets say you sign with a NYC Agent that Specializes in Feature Films shot in NYC.
A Great Exclusive for BOTH of you would be that you are "Exclusive to that Agent for all Feature Film work shot in NYC" (A lot will use "GREATER NYC AREA" or "TRISTATE AREA" to Incl Parts of Connecticut and North New Jersey but can lead to contract disputes because there is no 'DEFINED AREA" or will use Within 30 miles, 50 Miles etc of my NYC office - it includes the same area but provides a clear line)

That Agent then goes about doing their Job - Marketing your look and Your Abilities to every Casting Director Hiring for that type of work (Feature Films) within that territory (NYC, Greater NYC Area, Tristate Area, Within 50 miles of their NYC Office). They will get the Breakdowns of Every Film shooting in that area and Get your Look and Ability in front of the Casting Director for every part you fit.
You will pay them for ALL OF THEIR EFFORTS ON YOUR BEHALF by paying them the commission they earned even if the job actually comes to you by a different source and even if you ALSO paid that other source a Commission.

Examples of Bad Exclusives COULD BE:
1) does not Limit the exclusive to Feature Films. UNLESS the Agent ALSO is well known to the Casting Directors for other segments of the business the EXCLUSIVE covers with a strong record of routinely submitting talent to them and a PROVEN RECORD OF getting that Talent hired.
2) States Location of Exclusive as New York, Connecticut, Pennsylvania and New Jersey UNLESS the Agent ALSO is well known to the Casting Directors in Connecticut, Philadelphia, Pittsburgh, Atlantic City with a strong record of routinely submitting talent to them and a PROVEN RECORD OF getting that Talent hired.

3) INCLUDES LA in the EXCLUSIVE. Many will state they have an Associate there - You of course will be happy to talk to that Associate when the time is right and conceder them (along with others).
4) Wants to have the Exclusive Cover all work, worldwide This is fine if you are Martin Sheen, Denzel Washington, or Julia Roberts and dealing with Creative Artists Agency, or William Morris Agency But not to anyone that is not that level Talent.

Now your Question about multiple Agents

As a Talent Looking for Good High Paying steady work, it is in your best Interest to sign with as many Agents in as many markets you are able to work in.Most of the time each agent will have developed their own different sources for job notices and should not overlap with each other but IF 2 Agents talk to you about the same job - you must accept from the 1st 1 that contacted you and IMMEADIATLY tell the 2nd that you have already been submitted for that job.
Your Honesty/Integrity with your Agents can NEVER come into question - They can never feel you are using them against each other or favoring a Lower Commission Agent at their expense.

The NYC market almost Requires you to have Multiple Agents to cover all the different types of jobs you want to do - but the CONTRACT CLAUSES make it EXTREAMLY DIFFICULT and GREAT CARE is needed or you will create a mess that other contract clauses makes it difficult to fix. Several Philadelphia Based Casting Directors do have Contacts and Hire Actors for NYC based Productions. The key to an Agent in 1 Market Representing you in another Market is Simply "Does That Agent have good Sound Contacts in that Market, a Proven track record of submitting talent to those contacts and getting that talent hired."
Good Luck

This is so AWESOME!!! Great information!