You have to ask your broker.
There are several methods that could be used. Basically your broker has to track the individual basis of all your shares. But when you sell, they might sell your newest shares first, or your oldest shares first, or your most expensive shares first, or your cheapest shares first, or maybe some other method entirely. Sometimes you can specify what method to use. If you don't specify, they will use their default method, whatever that is.
