What is the limit to the number of GitHub organizations you can create in a single account? It doesn't seem like there is a public disclosure regarding the number of organizations that one account can register. So, is there a limit?
With most things on GitHub, if you don't see a hard limit published, you'll have to consider the main guideline of "be reasonable".
If it gets to a point where you're creating that many organisations, you might get a friendly warning email from support. They'll then work with you to determine if it's abusing their system or ask why you need to create so many orgs in the first place.
It wouldn't be any different really compared to their "soft limit" on repo sizes. They do suggest keeping things under 1GB, but if you go over that, they'll ping you and work with you to get it under.
Other than that, you should be okay to create as many as you need "within reason".