I really don't think there is a magic number of calcium demanding corals to have before needing a reactor. There are a lot of very successful SPS
tanks out there that don't use a reactor at all. As already stated, make sure that your CA
levels are where they should be and maintain that level and you will be fine. I still don't have a CA
reactor on my system and I grow a lot of SPS
Using a good salt mix and doing frequent PWCs to replenish trace elements (CA
included) will take care of your CA
level. When the time comes using a 2 part system to dose your tank will cover any increase in CA
demand. I'm using B-ionic now but am doing research on mixing my own 2 part solutions.