the 2310 has 3. What those pre-outs are used for is the subwoofer and a second zone. To contrast, the 4310 has 12, 8 for 7.1 HT and 2 each for two more zones. In other recievers that have pre-outs for the main HT channels, L, C, R, Surround L & R and Back L & R, those pre-outs will bypass the internal amp of a reciever if you have an external amp plugged in. What this lets you do, is, some day if you decide you want to upgrade your CM 7 to a speaker that is more power hungry, you can connect a 2 channel amplifier, or two one channel (mono) amplifiers to the L and R pre-outs, and thus meet the power requirements for your new speakers without having to buy a new processor for your whole system.
This is something that a lot of the people here do, but by no means is a requirement. I don't know for sure, but I think I've read a lot about Onkyo recievers in the same pricepoint as the 2310 that do have the pre-outs.