I don’t think there is a set number of inverts for a reef tank. Personally, I would start with a few snails for algae control and a couple nassarius snails for any leftover food. If you have inverts that are eating corals, dying without any parameter swings or heavily competing for food that is a good indicator that you have too many. A healthy ecosystem will balance out. If you have die off, it will more than likely cause an algae outbreak.