When I use Azir, Sovereign to attack a battlefield, do I have to pick the number of tokens I want to move before resolution? Or can I decide later on when the attack trigger is being resolved?

No — you lock in which tokens you're moving, and how many, when the trigger is finalized onto the chain, not later.

Azir's trigger reads "you may move any number of your token units to this battlefield," and that "you may" is the first part of the effect, so the choice to perform it at all is made during finalization (Rule 383.3.a, 403.1) — declining removes it from the chain entirely. If you do perform it, you pick the specific token units right then; "any number" just means you may pick any count of the legal ones, including zero (Rule 355.13, 403.2). Those choices are locked once the item is finalized (Rule 355.15).

In practice this means an opponent's Reaction can't change your picks, and you can't add a token you make later in the chain — a chosen token that's killed or moved off the board before resolution simply doesn't move (Rule 359.3.e.5). Only "this battlefield" is evaluated at resolution, so it's wherever Azir is sitting when the trigger actually resolves.