Well, it depends how many would be too many. If they are all exceptionally designed holes, then why not 18 of them? But if they were built on less than stellar property by a less than stelar designer, then 4 or 5 might be too many. But then again, it still depends. I could easily see something like a 100 yard par 3 (2 1/2), a 275 yard par 3 (3 1/2), a 310 par 4 (3 1/2), a 520 par 4 (4 1/2) a 550 par 5 (4 1/2) and a 690 par 5 (5 1/2) and those all make sense depending on where they fall in the routing. Add in some other interesting holes and those 6 could be very good. (Now I don't really think a 690 par 5, unless its very tricked up, would play a 5 1/2 for anyone who should be playing a set of tees with a 275 par 3 and a 520 par 4, but thats another matter)
But the main answer is I don't think there can be too many, so long as they are all designed well.