Really depends on what you want and what rock structure you have. I have a three column rock structure in my 40Br, softy dominated tank... I use a MP10 on 75% power plus my AC500 on 100% power. Works for me but for most it will be a lil' under. To get a lot of the spots for a more complicated rock structure you may need more.
I have an MP40 on Reef Crest mode (100 % ) and a Koralia evo 1400 with SPS and a grip of LPS. Its a tasty combo but my fish all seem to hang out on the opposite side of the tank as the MP40 or in the cave underneath it, its cool though cuz I have yellow assessors so they chill upside down all day and my clown wont leave her RBTA unless we're at the tank.