The more the merrier. I think the most important thing is to have an acceptable balance: a park that has one superb coaster to 30 other rides isn't all that nice for an enthusiast, on the other hand 15 coasters with NOTHING else would be no fun either (or less, at least). For the prices that most amusement parks charge ($30-50) I think 6-9, depending on the quality, but tending toward more in order to keep lines shorter.
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Po!nt of View: A different look at Roller Coasters.
http://www.crosswinds.net/~justmayntz/thrills/index.html