when i mean few i mean an opening for your white game; a defense vs 1.e4 as black and a defense to 1.d4 as black
playing the same openings and improving and playing better players will sharpen your openings
i play 1d4 and almost always 2c4
i play ...c6 vs 1.e4
I never said you have to MASTER it.They need just to understand the basic concepts like whem they can push d5, castling opposite sides attack etc..