How can I combine submission counts to know when a course is full?

We have registration forms for 3 courses, each with a specific limit on number of students. And we have at the moment a Black Friday bundle deal that offers a pair of courses for a discount price. 

So we have registrations arriving for each course from 2 different sources, its own form and the bundle form. We want to cut off registrations when total registrations from both sources reach a course's limit. And we want to eliminate a bundle option if either of its courses fills (and instead show SOLD OUT).

I can easily set up the conditional logic IF I can access in one form the submission count of another form. That submission count is, of course, not a form field but a system field. Is this possible?

Unfortunately, it is not possible to access the submission count of another form so what you can do for now is to implement a submissions limit on each form.

Guide: http://www.jotform.com/help/137-How-to-Limit-Submissions-in-a-Form

You might also want to use this widget in your form https://widgets.jotform.com/widget/submissions_counter. Use this if you want to display the number of submissions or remaining submissions of a form.

By the way, if you know API and javascript, you might be able to achieve that.

https://api.jotform.com/docs/

You can get the number of submissions of a form from this endpoint https://api.jotform.com/docs/#user-forms