You only need one opening as white

You only need one opening as white. Really more of a statement to be pondered than anything else. Cases where you clearly need more than one:

1) You are playing the same person over and over

2) You are a GM/people study you

Other than that, it seems to me you only really need one. I played Queen's Gambit and got to know it quite well, and only switched over to King's Gambit because I wanted something a bit more aggressive (and was getting a bit bored with QG). Thoughts?

If you play the King's Gambit you're playing 1. e4 so you have to know what to do against the Sicilian, French, Caro-Kann, Scandinavian, Alekhine's Defence, Pirc, and possibly some lesser-known bits and bobs such as Owen's Defence and the Nimzowich Defence. It's only 'one opening' in the loosest sense of the word!

What if your "one opening" is the Queen's gambit and they don't play 1...d5?

Indeed, then you need to know what to do against the Nimzo indian or Queen's Indian/Bogo Indian, the King's Indian, Grunfeld, Benoni, Benko Gambit, Budapest Gambit, Modern Defence, English Defence (possibly), Dutch Defence, and again, a range of minor alternatives. 

Hmm yes that is true. You are correct. But, could we agree that one's repetoire as black has to be substantially deeper, given that the other guy makes the first move?

You, you uh... improvise...... okay, point taken.

I also know only one opening Queen pawn

You're right, but in compensation for the depth you can keep it very narrow. For example, you could play 1. ... c6 followed by 2. ... d5 against everything which means you only need to know the black side of the Caro-Kann, Slav, and (occasionally) Reti. 

Equally, you could meet everything with the Modern Defence (1. ... g6 and 2. ... Bg7 and 3. ... d6). 

Or heck, you could just go on general principles: occupy the center, develop your pieces, knights before bishops, castle early, etc.

Voila, you don't need any openings as white or black.

the english has a tendency to transpose into other openings, but often you can stick to just that and be just fine

If you want a rating like yours you don't! :p

1568 is a good rating... a very good rating.

I'm a work in progress. I expect my rating to improve.

However, my previous comment was tongue in cheek in the light of the relentless campaign of some posters to convince newer players not to study the openings.

I think everyone should study the openings, but it's a tricky business to know how much you should for your particular level and situation.

I don't think it's ideal to have only one opening as white or black. It might work for some people though.