I try to buy as many as possible, depends on what's available, and what I can afford.
Last year I bought 10 belly crawler pikes, and the largest female ate the other 9.
I bought 4 Geophagus iporangensus this spring, and within 2 weeks I had a spawn.
If at all possible, somewhere between 6 and 16 (although if the price is right, I've been know to pick up 30).