with a charger with a max of 2 amps, how high of a mah capacity can it charge

The 2 amps only limits how long the battery takes to charge, not the maximum capacity you can charge.

For example, a 2000mah battery would take 1 hour to charge on that charger. A 4000mah battery would take 2 hours, a 6000mah battery would take 3 hours and so on...

What kind of charger? Trickle or peak or?? Guess this is AC and not DC charger?

Trickle type charger could charge pretty high mah - just matter of how long stays on charger. I have a 2amp car trickle charger and it'll charger an auto batt given a few days time.

Usually peak type chargers have a limit that they will handle thats based on max voltage charger can output. Its not directly related to the charge current, but of its limited to 2amps, it mighht not charge high capacity.