Has Jean Luc Picard ever said, "I have to go, Number One (or two)".?
Based on a quick search of Chakoteya's excellent script search, at no point in Star Trek history has anyone uttered the words...
...or any variation thereof. And while Picard has uttered the words...
...on no less than 13 separate occasions, none of them are in conjunction with Number One
That being said, you may be thinking of Q's rather funny quip in TNG: True Q. At the time, Q was wearing a Starfleet Captain's uniform.
[Riker ENTERS and stops when he sees Q and Amanda. Q is standing in front of her... her eyes are closed, as though concentrating on something. The minute Q sees Riker --]
Q: If it isn't Number Two...
or you might even be thinking of Beavis and Butthead (warning, NSFW content)
(Not)Picard: Number One, I order you to go take a number two.
(Not)Riker: Aye Aye, Captain.
(Not)Picard: Hehe. [Reflective Pause] This is cool