I wonder how many master nodes will work after piex swap is over.

I am betting that all of the PIEX nodes will work . . the number of nodes though? That should be interesting to see.
I'm going to reiterate for anyone who has not swapped from PIE to PIEX . . the process is smooth, easy, and painless. Get on the new chain guys, no reason to stay on the old one at this point. PIEX is going places.