I used 2
I've heard rumors that with caution, you can do both tanks with one quart.
I threw caution to the wind and used about 1.25 - 1.5. You'll need some on the fuselage for the firewall, cooling ramp, and NACA vents also. Oh, and after doing the tanks, you start thinking of other uses for Proseal.
Get two, store the one you're not using in the freezer.