How many PROJECTS

Not sure I understand your question.

Opening a project opens its timeline only. So in that respect, the answer is one.

OTOH, here is no limit to my knowledge of the number of project timeline content that you could paste into a single timeline.

Or are you talking about project timeline history?

Please provide more details of what you're trying to accomplish.

Russ

JJust to add to Russ' message: there is no limit to recently opened projects or timeline items in the timeline history menu.