For vocalists, I find there is a curve on "takes" where often it's the 2nd or 3rd take that is the best, and then it starts going downhill from there.

Where that peak is, is different for everyone, but my experence says that after about take 4 or 5, it's not going to get any better in that session. If we don't nail it by then, I have the artist go home and practice, then come back in the studio.