How many contracts using market orders can one safely trade in EMD, CL, and 6E, during CME Index RTH without incurring more than a couple of slippage ticks?

Don't know about EMD, but with 6E and think with CL too u will not have this type of problem, unless u trade hundreds of contracts, but u can incur in slippage more due fast market conditions, and more on CL cos sometime it jumps some ticks.

LukeGeniol 

Don't know about EMD, but with 6E and think with CL too u will not have this type of problem, unless u trade hundreds of contracts, but u can incur in slippage more due fast market conditions, and more on CL cos sometime it jumps some ticks.

Are there 00's of contracts on both sides most of the time?

Hard 8 

Are there 00's of contracts on both sides most of the time?

For 6E on London and NY morning session, for CL on NY session.
Just take a look at DOM for some days.