Clown police on patrol......
The real answer is that it really depends on the clown species and the clowns themselves. I have 3 percula clowns and a pair of clarkiis in a 20 gallon. They each have their own anemone....a haddoni and a small gigantea. They get along just fine and have now for the last year and a half. They were all introduced together and while there is the occasional posturing, they never bite each other and all feed together fine. I plan to take them all home in 8 months to my 155.
Now Tomato and Maroon clowns would likely never get along like that with other clowns or others of their own kind. But I have always had good luck with percs and ocellaris. Saddlebacks and pink skunk clowns tend to be pretty peaceful too. Clarkiis can get bad sometimes, but I figure that most of their life, they have been with the percs and while now bigger, they get along fine.
So, all in all, it really depends on what kind of clowns you are dealing with. The only reason I even tried to put them all together was because if it did not work out, I only have to walk 100 yards to release whichever clowns I no longer can keep. Without that luxury, I would make sure you have a backup plan if things so not go smoothly. You could have the problems cited above by the others, but it is not a concrete or definite rule......more of a principle to be aware of.