How many you start with is really your choice. With a 200 gallon tank, you could certainly double it and go for 48. Or, if you keep them happy, then the size of your group will increase on its own.
Personally, I would rather have (2) 100 gallon tanks then I could have two groups!! Sorry its that breeder mentality I have! I would be too cheap to buy 48, and would probably just let the group grow on its own. I don't think your tank will look empty with 24 Tropheus in it, it will just give them more room, which lessens aggression, more area to graze on, which lessens aggression, etc.