I'm using Minwax fast-drying polyurethane to finish some new stair treads. How many coats should apply for a durable finish?

Check out the following link:
http://www.minwax.com/products/protecti ... r-fast.cfm

I typically use 3+ coats on my floors, though they recommend 2-3. You will probably be able to tell if you need subsequent coats by the way the stairs look & the specie of wood. If you stained the treads first it may affect the number of coats also. If you can see lap marks or "beads" you can always buff those out with e.g. 220 sand paper and put another coat on until you're happy with them!