As far as I know there is no "limit" other than the number of controls on a form. But there is a limit of 7 nested subforms (subforms within subforms). And yes, they are linked to the main form by the Master/Child property.
I have a form with a dozen popup subforms, which in turn have well over 30 popups within the subforms, and they can all be open at the same time. Only problem with that is ... form management when closing a subform, you may end up on a subform that you did not intend. You may want to include a close all subforms command button if you are planning a large number.