How many initial moves are possible in Chess, 20 or 21? Each pawn, of which there are 8 white, can move 1 or 2 squares forward.
Each knight, of which there are 2 white can move forward and left or forward and right.

That gives 20.

But white can also resign, and in some tournament situations this might be sensible.

But is resignation a move?

I sometimes attend quiz nights and this is a common question, and the answer is always said to be 20. I,  however, always say it is really 21 because of the resignation option. But, is resignation actually a move?
Your question essentially reduces to asking whether resignation is considered a move or not. To be exact, you are asking about legal moves, the topic of which is addressed in (and defined by) Article 3 of the Laws of Chess. Resignation isn't mentioned there, so it's not a move.

A heuristic argument: you are allowed to make a move and then resign immediately, i.e. if resignation was a move, you have effectively made two consecutive moves, which is not allowed.

Or more simply: since you can resign even when it's not your move, it follows that resignation is not considered a move.