I don't know what a jgroup is. I assumed you were just talking about Joomla usergroups. Sorry at this point I've no clue what you're talking about. We're not the developers of joomdle and we've no integration with joomdle so we can't help you with any of that.

There isn't a hard limit. Joomla usergroups and view access levels are apart of Joomlas ACL. Joomlas ACL is basically used everywhere. The more usergroups you have the more performance you're going to lose due to this. It's also going to make configuration of permissions nearly impossible as it's going to be a massive input list that'll probably hit input post limits.

I don't know if Joomdle has a feature to disable creating usergroups like that, but you may want to contact them regarding this as it will become a serious problem if you continue to create dozens of usergroups. Ideally Joomdle should be utilizing its own custom database table and implementation for checking course access instead of creating a Joomla usergroup with every course.

If you don't expect your usergroups to climb over more than 100 you should be fine, but if it's going to continue to climb well above that you'll want to evaluate a potential solution now to avoid a massive headache down the line.