It depends. How much does your round bale weigh and how much do available square bales weigh? Hay cannot be measured by size, it must be compared weight to weight.

If you have a 500 pound round bale and 50 pound square bales, then 10 square bales would equal 1 round bale - you'll seldom find the math that convenient in the real world, though. Even bales from the same farmer, same cut, same field will vary in weight.