One account per service per region per deployment stage (e.g. dev, test, production) is the best practice. How you define "service" is up to you, but I usually divide them by trust/dependency footprints.

This significantly reduces the risk that an account-level problem or throttle will disrupt your entire business.

Fortunately, AWS finally seems to be getting serious about building management tools for multiple accounts. AWS Organizations is way better than the old consolidated billing stuff.