You can never level up enough early on in the game - Sid Meier
As a general rule, you should looking to start off easy, letting the user level up quickly without too much effort. This gets them into the swing of things, gaining confidence and achieving momentum.
As you further develop your chosen topic it’s likely that you will need to explain concepts and principles in more depth. As such, later levels will often be harder to complete. This could be because there is simply more content, or it could be because you encourage more participation as the course progresses.
Whilst there are no limits to the number of levels you can create in a social learning experience and no limits to the number of objects you can have at each level we would always advise adopting a critical eye over the size/length/duration of your course to ensure that it doesn't become too broad; in which case you’d be better off creating multiple courses and splitting up the content.