Maximum slides in a Captivate project

I would continue to keep them in separate project files. The trend these days is to allow the LMS to manage a curriculum for you, rather than trying to get hours of training into a single Captivate project file. Learners appreciate a more modular approach. If they have a few minutes to kill, they can complete a portion of the course rather than feel compelled to go through everything. It also addresses cognitive load in that completing many smaller modules makes it less difficult for your learners and gives them a sense of accomplishment when they complete each smaller lesson.

That was not a Captivate problem but a problem with bandwidth. Bandwidth is now much higher but if your learners also use mobile devices which are much more limited than desktop/laptop and maybe also on a lower bandwicth? HTML5 output is like a website, can become very complicated. There is no unique answer to your question...