I have a 75g tank woth 1 mp40 and it is plenty of flow but I'm now going to buy a slave pump for my tank so the modes function as intended. I'm deciding between a mp10 or a mp40 for the slave pump, the deciding factor is that if I ever upgrade in size of tank I would want an mp40 but an mp10 would be cheaper and suffice for my 75g. So basically 1 mp40 is def enough flow but for the pumps modes to function as intended your going to want two. I'd suggest 2 MP40's if you ever plan to upgrade in tank sizes or 1 mp40 and 1 mp10, be sure to buy the w verisons so they can operate in slave mode with each other.
I will most likely buy 2 mp40's for my 75g so that way if I ever upgrade in tank size I'll be set to go. I know its pricey but well worth it IMO.