How to calculate coverage in recommender systems?
Right, the capital letters denote the total available. In that blog post: U means all users, N means all items but in other places is usually written I, and L means all top-n recommendation lists. "Top-n" means that the recommender system outputs a ranked list of n items, so if you had 1000 users all getting a Top-10 list, you'd have L length of 1000*10.

I suggest you read Ge, Mouzhi, Carla Delgado-Battenfeld, and Dietmar Jannach. "Beyond accuracy: evaluating recommender systems by coverage and serendipity." Proceedings of the fourth ACM conference on Recommender systems. ACM, 2010, at http://ls13-www.cs.tu-dortmund.de/homepage/publications/jannach/Conference_RECSYS10b.pdf instead. It's a more clearly written definition of metrics and uses more traditional recommender system notation.