the 2800 series will scale to 12 T1/E1s while the 3800 series go to 24 T1/E1s
I believe the new 2900 and 3900 series will support even more then their equivalent 2800 and 3800 models with the new PVDM3s.
Just to add some comments , I would not overload any GW even if it could support more. From a DR point of view you are putting all your eggs in one basket. By having more then a single GW you increase your DR capability, have different cabinets, different comms room etc. Also if you have to power off the one GW you lose all circuits- The multiple GW`s approach is more expensive but may be better long term?
One more item you might want to consider and look into -
Cisco just introduced the next generation of ISRs - the 39XX, 29XX, 19XX, etc.. The way things are EOL'd these days you want to protect your investment. Of course the new boxes are more powerful too.
As an addendum to my post yesterday and to re-iterate what Art and iptuser55 have added, when you take these routers into the real world and are looking to fully utilize these E1s it's worth taking into consideration what the CPU of these routers can actually accomplish with regard to Maximum Simultaneous Calls rather than just the number of supported E1s
If you believe that you're going to be running these E1s at close to full capacity you're looking at 360 simultaneous calls. The only router in the x800 series that can do this is comfortably (ie at less than 75% CPU) the 3845 if it's not performing any other fuctions, eg a WAN gateway. In the x900 range, the 2951 and up will do the trick, but as iptuser55 said, you wouldn't neccessarily want to be running these at these loads without redundancy.
If it's possible to split the load into two ISRs you could do that or have a look at the AS5400XM universal gateway.