Zero is good enough. Good middlegame play > good opening play. Simply by playing a few games of "experimenting", you'll naturally figure out all the theory you need to know. I used to read (ok, I still sort of do for entertainment) books on opening theory. It didn't really help my chess, opening-wise. After I decided to focus on middlegames instead, my rating jumped up about 200 points.
On the other hand, study what you like! I was always fascinated by the history of how different openings developed, and enjoyed going through opening books with the purpose of learning how masters came up with "prepared lines", as well as how they tested their ideas against each other, then went back and made small adjustments to their ideas and try again.
For an inexperienced player like myself who is approximately a 1400 player, what is a good number of openings to know well to advance my abilities? There are so many openings that it feels overwhelming. However one needs to be able to appropriately respond to their opponents moves.