Cherries breed like rabbits; so I would most probably say to start off with 6 and if you have both sexes, planted tank etc, you'll have loads within a month x
I agree with Beth, not too many. However IF you can guarantee different sexes then I'd say 5 (or whatever number the shop do a deal on, if they do). I bought 5, all females! then I got another 5, 4 females and one male...