the default is 16 connections, this can be modified with the max-compression-connections command.
Most version do compression in the fast switched path so it's not too intensive, but there are some bugs associated with ncertain interfaces where it is neccesary to run process switching, that's where it gets tricky. It is hard to say since I don't know what else the router is doing, the best bet is to start putting cals across and keep an eye on the CPU.
With no other loads, I would guess that the 2620 should be able to handle all 24 calls if it runs cRTP in fast switch mode. Run sh ip interface to see the switching mode for the interface.
Open a TAC case if you run into issues with this.
