There are no limits for the total number of modules you can add to a Resource Center. However, we don't recommend adding too many of them.
As a best practice, you could group 5-6 modules on a single Resource Center related to a specific topic.
As an example, here, you can see a Resource Center that focuses only on the User Onboarding subject.
If you want to talk about different subjects, you can create multiple Resource Centers. Then you could add the correlated and respective modules to each of them.