Hi im super new like never owned a salt water before so how big of a tank for 5 BC's and how established should the tank be
You may be able to do it with 150g. The reason is that in most cases you will have REALLY hard time keeping more then two in an aquarium assuming those two are paired up. If they are not paired up then you will end up with one in most cases. In short stick with just two and do your best to buy two that are already pair up in the store. Or there are some online sources that sell paired fish.