from my understanding of MW, the question of how many worlds can be answered pretty well by ~2 to the power of the average number of decoherence events since the beginning. Unless there's some wierdness with a lot of worlds getting terminated or still-lifed early.

The difference between counting the states in a quantum computer (for example) as one world or many is at most a constant factor, so the fuzziness on our concept of "world" isn't actually that much of a big deal. (I chose a quantum computer because it is probably the most definition-stretching phenomenon).

barring weird stuff like quantum computers, branches get very separate very fast, so I don't think it's all that weird to talk about number of worlds.