Tostacho

In my small town in Mexico, we just have one casino and they have just one type of cash game, it is NLHE (blinds 1.5/3 usd. aprox) and all the players gets in the game with 20bbs. Later when they loose, they re-buy same 20bbs over and over. They are very bad players, they have no idea of what are they doing. Well, the question here is... How many bbs should I buy? or if you think that I have to wait until the stacks become larger?