I’ve never seen a community that caps members’ post count but I can say that I’ve blocked people that have hogged the activity feed due to the sheer wall of content they’ve posted. Most people didn’t come here for a blog-type experience.

I don’t recall seeing a post rate limit, just community limits.

No limit, post away. Anyone who cares isnt worth thinking about

It would be possible to fall foul of annoying, harrassing, content-free, “Should have just hit ‘Like’”, and other such rules and conventions of the subreddit. A good starting test for your sister would be “Would I like it if every other member of it did what I was doing in my favourite subreddit?”