Welcome on board. Turbo snails get large. In my opinion one is enough. And watch them. They like to call and land on their shells. And they pay there screaming " help I've fallen and can't get up". They face drowning if not helped. Not trying to talk you out of one. I have two and really enjoy them. They also can live years. And they also knock coral off.
I like to do fish first actually. In a new tank, there isn’t much for the CUC to eat nor is there really a need for them, other than aesthetics. If you choose your CUC for an actual purpose, it’s easier to choose the correct kind. Once you add a fish or two, you start feeding the tank and then you get nutrients and you’ll see algae growth and who knows what else. You can then add CUC based on needs. Hope this helps!