Generally speaking, you will not be able to keep more than 2 clowns in any tank.
Clownfish all start out as unsexed juveniles. The most dominant one will turn into a female, and the next will become a male. A male can later become a female, but they can not go back down the chain (undiff -> male -> female and it only goes that way). Apparently, the male stage can be skipped as well (with a direct undiff -> female). All others stay undifferentiated. Eventually, the undifferentiated ones want to spread their genes. In order to do so, the present male or female needs to be driven off (or the undiff one needs to find a new home, which it can't in a tank). This leads to constant fighting and typically death of all but 2 clowns.
They can be peaceful for some time, but eventually this seems to happen to all clownfish keepers.
When you do get 2 clownfish, get the smallest ones you can find. The larger ones, especially those kept alone will likely be female and you won't be able to combine two females. The small juveniles are almost guaranteed to be undifferentiated and will sort it out relatively peacefully on their own.
Oh, and two should be okay in a 40 gal. You can mix the normal patterned ocellaris and B&W ocellaris (1 of each, not more than 2 clowns total). They're just different color morphs.
