There are 3 to 6 keys per act. Each key has a specific level, and opens up to that dungeon. There are 2 level 1 BF keys for the first act, one hard to find and the other hard to miss.
For act 1, once you find the first key, you can enter the BF and the first dungeon. The other dungeons will remain locked until you find higher level keys later in the act.