1. By the bottle. Bottles are usually 750ml, but 700ml and 1000ml are not that rare. In any case, you bought that bottle of that specific wine brand/year. The quantity is whatever the producer decided. The price is per bottle and quantity doesn't factor in.

2. By volume. This is only used when you buy in bulk. You go to a winery where they have huge barrels with a faucet. You can taste the wine and then buy any quantity you want (you can even bring your own container).