You should know one opening against every move you face regularly. Usually this means as black knowing a response to e4, and a response to d4. As white it depends on your choice of first move, but for example an e4 player should probably know a response to e5, c5, e6, c6.
In general knowing ideas/principles is better than knowing specific moves. If you are going to memorize opening moves I would say probably at most 5 moves of the opening and then a general plan.
At one time I played 5 different White openings and 4 Black defenses. Now I've reduced that to playing with 3 White openings and 2 Black defenses. I'm thinking "less is more;" that it's better/easier to really "know" a smaller amount of openings/defenses. Right now I play OTB with a small number of different players, so I worry that if I play only a few openings/defenses these players will be "ready" for me and will be able to anticipate my moves. I don't try a lot of variations in my openings. I like "set up" openings like the Botvinnik formation of the English opening, the Larsen opening, the Veresov opening, the Pirc defense and a Reverse Botvinnik system. What do you guys think?