How many chess opening should one know?

How many chess openings should one know for playing black and white?

You should know the barebones of everything. It helps you pick up concepts that you can apply to your own positions or positions that have left the typical move order (eg. "if I move here then I have an improved English). It also means you can follow analysis and have a clue what is going on in other games. There is no reason why you shouldn't know all the major openings and themes of them (even if it's three moves in some cases). 

Just one. Fischer and Kasparov played Sicilian and King's Indian Defence as black and Ruy Lopez as white.

At low level opening principes are most important, at a higher level i would advise at least opening for White, and the response against the most user defenses and pick 1 opening against ,E4 and 1 against D4. For example I play D4 mainlines as White, and know at least one Line against qgd, slav, KID, and many more, i know best what I face slot at my club. For black I know E4 E5 against e4 and the QGD, against D4, its only moe that om learning some sharper openings for black like najdorf and anti sicilians against e4 and grunfeld and some sidelines like against London against D4

I would throw the number of 3 out there.

I play one opening as white, and 3 defenses as black to confront e4, d4 and c4.

Of course this depends on what level you are at and how far you want to climb. I know club guys at the 1800-2000 level and they'll exclusive play e4 as white, when black: 1.e4, e5, 1.d4, d5, 1.c4, e5.

I know one for white and 2 for black

I play 6 because I'm indecisive

ok true, c4 and Nf3 are also good openings, though not as common

As White you need to know the common defenses to your opening move. As Black you need to know a defense to 1. e4, 1, d4, 1.c4 , 1.g3, 1.b3, and 1, Nf3.