How much ram shall I dedicate to minecraft?

If your talking about you hosting a server you should check out canihostaminecraftserver.com it tells you how many people you can host on your server. But if we're talking about singleplayer/multiplayer with you not hosting you don't need around 1-2 GB for the minimum quality you are just in the middle. If I were you I would use somewere maybe even less around 1.50 GB.

I only use 1GB for my server and it works fine.