In theory maximum 500,000,000 BOS would be available for trade, but in reality it would be around 410,000,000 BOS at maximum, as that is the amount of BOScoin that has been allocated to owners who have participated during the (pre)fundraiser event.

Considering the incentive to run a node, which requires at least 40,000 BOS, a fair amount of people might also hold on to their BOS, and will most likely result in a circulating supply of less than 410,000,000.