See 5.1.1.1 & 5.1.1.2 on https://pages.nist.gov/800-63-3/sp800-63b.html. Eight minimum, accept at least up to 64 characters, forbid breached passwords, dictionary words, aaaaaaa style passwords, and usernames, but beyond that:
> Verifiers SHOULD NOT impose other composition rules (e.g., requiring mixtures of different character types or prohibiting consecutively repeated characters) for memorized secrets. Verifiers SHOULD NOT require memorized secrets to be changed arbitrarily (e.g., periodically). However, verifiers SHALL force a change if there is evidence of compromise of the authenticator.
It's simple to calculate the actual entropy and check against a list of common weak passwords (large lists are easily obtained and kept up to date)