[Sharlin]

You probably mean inbreeding. Interbreeding is good. It is good that the bison herds mingle and interbreed.

Anyway, for mammals an initial population of a couple dozen individuals (assuming they're reasonably genetically diverse in the first place) is plenty enough to produce a population of any size without problems.

[onychomys]

There's a general guideline called the 50-500 rule. You need at least 50 animals to avoid immediate inbreeding (and also stochastic extinction from a fire or flood or disease etc), and about 500 to have a genetically healthy population. That varies some after a bottleneck event since your genetic population will be functionally less than your actual physical one, but it's a decent way to approach the problem.