Are you doing your full summer at one firm, or splitting?
For 2Ls, I expect the firms to average giving offers to 1/3 to 1/2 of the people they bring in for callbacks. With 6 callbacks, that is a possible 2-3 offers. They'll probably be weighted towards the "lesser" firm, meaning those who are not as elite in their interviewing process.
If you're doing the full summer at one firm, then 2-3 offers is a decent number to choose between. As long as you don't bomb the callbacks and end up with 0-1 offers, that's fine. If you can manage it, try for 8. It gives you a bit more buffer. It also lets you have more points of comparison in making your decision.
If you're doing half summers at two firms, then 2-3 offers means you don't really have much of a choice. I'd try to manage 8 interviews as a minimum, and cap out at 12.
This ignores the problems of doing more interviews, though. You'll have to miss more classes. That's my gut opinion. Listen to your own gut more, of course. Just remember that this is your prime interviewing time. Interviewing next semester or next year will be more difficult, as people will start out wondering what is wrong with you that you don't have a job lined up already.