If you have terrific grades, stats, and seem to interview well by getting a lot of callbacks, assuming you have callbacks at firms you think you like, I wouldn't accept every call back out there. It is ok to be somewhat selective. Think of this like college applications, though, you will want to have reach firms, firms you stand a good chance of getting in, and safety firms where you think you should get an offer. Now of course, there's no telling what happens. Sometimes one interviewer just won't click with you and will ding you hard, even when you thought you'd breeze through an offer. If your grades are so so, take the callbacks -- all of them -- unless you have absolutely no interest in the firm/city. You need to cover your a**
How do we figure out how many callbacks to give? It really depends. If you are from a local school, it is usual easier to get a callback because we don't factor in travel expenses. And, we probably have a good connection to the school from previous hires. For me, it depends on how we are doing offer-wise; if we have several offers outstanding, I will be tighter on approving callbacks. In terms of offers given for each position, this depends on each firm's usual yield and number of slots in the office. For some firms, you might give 3-4 offers per position or more if not such a "hot" firm (we analyze historical data). In smaller programs where you are only authorized for a few summer associates total in an office, we have to be really careful about oversubscription, so for say 4 slots, we might give 6-7 offers depending on how we sense interest.