I have a 75 had it running for a couple of years with only one. And my tank is sps dominate. Just recently added another one though. I think one will work until you can get another one.
I have one on an sps dom 75 and I love it, but I agree that the amount and size of the sps and especially the aquascape matter so much. I will be looking to add another one in the future, it makes it easier on the wallet. lol