It doesn't matter how many systems are in there - as long as they are immersive, fit into the world and have a purpose in a story. What really makes a good RPG is player's actually playing role in the world and making decisions that have impact to an extent.
How one learns to create things like that? You mean whole games or just certain mechanics?
In case of certain mechanics - what we do is implementing parts of gameplay mechanics as mini-games and testing whether they are fun for players (no magic in it - you literally make a small game doing that single mechanics - and let actual players play it and complain to you). The feedback from random players is important, because you need an opinion of people who didn't participate in design/talks/development of certain mechanics.
In case of whole games - it's well… complex topic, much bigger than what a single post could cover.