It depends on how it's implemented and I've never used the reddit API, but I assume it's just a single API call every time new posts are loaded. So it'll load a batch of posts and then once you've scrolled down far enough it'll make another call to load more. But basically everything you do in the app that interacts with reddit causes an API call, e.g. open a post/load comments, upvote, post or comment, view a profile or subscribe to a subreddit. Depending on how the API is designed, multiple calls may be needed.