In D&D, if you've got a base attack bonus that's high enough, how do you tell how many extra attacks you get?

Answer 1

Whenever your lowest base attack bonus reaches +6 you get a new attack at +1.

Examples when you gain another base attack point and had +5 your new base attack bonus is +6/+1. When you add to +10/+5 it becomes +11/+6/+1.

Also, all the Wizard of the Coast DnD stuff breaks down the tables for each character class. Independet stuff depends on the publisher.