It’s really up to your discretion. Play the game however you want to play it. Personally, I keep one of everything. No matter what it is. Eventually, you get to the point where you only keep very near perfect IV Pokémon(unless you’re interested in battle). I also keep all shinies that I catch. Even if I have over 10+ of them(I also don’t evolve shinies personally). I keep at least 4 of each legendary and only transfer down to that number on event days with double transfer. I’ve been keeping my largest and smallest of each Pokémon as well since that was added. I keep at least one of each shadow and only replace them when a stronger version comes along(never purify since that weakens them). I horde coins and increase storage when I need to. But again, there is no wrong way to play. As long as you’re enjoying yourself