Hello @Matt Dorsey ,
Adding users to Azure AD is free.
The cost will occur only if you decide to add a license to that specific user. This might depend on users' needs (if they need to use e-mail, or maybe access to configure advanced features.
The maximum number of users in a tenant is 50000, but this number might be higher depending on the subscription and it can also be increased on request.
For more information about limits associated with users and applications, you can check out this document:
https://learn.microsoft.com/en-us/azure/active-directory/enterprise-users/directory-service-limits-restrictions
Please let us know if you have more questions. If this answer was helpful, please click on Select Answer to help other users find this answer.
Thank you,
Vukasin