when i attack using azir sovereign, can i react using en garde first before moving my other units to the battlefield where my azir attackks?

Yes. En Garde has [Reaction], so you can play it in response to Azir, Sovereign's attack trigger while it's still on the Chain — it will resolve before the trigger moves any tokens.

This is playable during the showdown's initial chain too, since only Reactions are legal until the attack/defend triggers resolve (FAQ #4597). If Azir is the only unit you control at that battlefield when En Garde resolves, he gets the conditional extra +1 — the tokens haven't moved yet because his trigger is below En Garde on the Chain. Then his trigger resolves and moves them in.

One note: which token units are eligible to move is locked in when the attack trigger is placed on the Chain (FAQ #2127), so En Garde can't add or change those choices — it just gets to resolve before the move happens.