My 1 round (2 x 120 or 1 x 220) is about the same as 6 sheets of 4x5.

Very roughly speaking, a 120 film (6 cm x 83 cm) = 498 cm^2 times 2 = 996 cm^2, 4x5 film (10 cm x 12.5 cm) = 125 cm^2 times 6 = 750 cm^2, so 6 sheets of 4x5 is actually 25% less film area than what I consider 1 round's worth. I'd say you can do 2 rounds (12 sheets), if not 3 rounds (18 sheets) easy. Increase dev time by 15 sec for each additional round.