An independent tutor I was working with required 10 Actual LSAT's. This is the first of the Actual LSATs series. Obviously the more practice the better, but there are some things to consider. I found that the logic games in 10 actual were much easier than the games from later years. I felt very confident doing the games in 10 ACTUAL, but when I hit the later games, I just couldn't get it.
More recent games will give you a better sense of what you'll actually be facing on the LSAT. I got all three of the released 2003 tests and it helped a lot.