learn just one opening?

Well no.  I mean you could choose as white to always open with e4, the king's pawn opening, but then black gets to decide where to go from there...

No, aopenings are also rwsponses to your players moves(when black). Its alwaya good to know more than one opening. I recomend you learn: scholar mate, sicilian defense, scandonavian defense and the queens gambit

maybe you meant one opening as white and one opening as black. you can. it is harder to make the case for black because the openings you can play vs 1 d4, 1 e4 and 1 c4 etc are not exactly easy to play and mostly have a bigger concession to make than your classical openings. i could name a few white system openings. the colle, london, the g3 english, the reversed queen's gambit tarrasch, the reti, the king's indian attack, the fianchetto with d4 and c4 vs all, to name a few. what i don't like about these openings is that there is not much tactical fighting until a later point, but starting out requires a lot of experience in tactical fights. personally i wouldn't recommend playing system openings until maybe you have some experience in open positions and more tactical stuff. but to each his own.

It depends on the Opening. For example, Wolfgang Uhlmann played the French his entire life, and thus, knew the resulting positions extremely well, which made him extremely tough to play against, even if you prepare.