Is there a limit to how many user can share a form with the account?

There is not a limit on the number of times that a form can be shared, it can be shared unlimited times, also when setting up sub-user accounts, there is not a limit on the forms that you can share with a sub-user. 

The only limit is the number of sub-users that an account can have, please check this guide about sub-users to get more info: https://www.jotform.com/help/232-How-to-Share-Forms-with-a-Sub-account-User 

You may also provide us more details about your question, we will be glad to help you.