during a chain resolotion i drew a defy from stupefy, can i react mid-resolution with this defy to caounter another spell

Yes — as long as another spell is still on the chain when Stupefy finishes resolving.

Stupefy is a Reaction spell that draws 1, and chains in Riftbound resolve one item at a time, not all at once. After Stupefy resolves, there's a priority window before the next remaining chain item resolves, so the freshly drawn Defy (also Reaction) can be added to the chain and will resolve first (FAQ 7449, 6797).

Two things to check: Defy needs a legal target, so it must be aimed at a spell still waiting on the chain — if Stupefy was the last item, the chain is empty and Defy can't be played. Also, Defy only counters spells costing 4 or less energy and no more than 1 power, and it checks printed cost (FAQ 5039).