the ink blot analogy is not quite so good for counting worlds because it implies more ambiguity than there is.

In reality the most ambiguous amplitude-blot is the inside of a quantum computer. The difference between considering that as all one world or many not-quite-decohered worlds is at most a constant factor on the exponentially growing total number. Most different worlds are very much distinct. The amplitude blots are quite small (atom scale) and infinite-dimensional configuration space is huge. All it takes is one photon to have gone a different way and the blobs are lightyears apart.

Assuming all branches are intact and active (as implied by conservation of amplitude), the number of worlds is approximately k*2^(r*t) where k is your strictness of what counts as a world, r is how many decoherence events happen per time, and t is time. I chose a base of 2 because all decoherence complexes can be approximately reduced to single splits. r can be adjusted if some other base is more natural.