"I was thinking they would be around the 40 to 50 # range. I guessed that only about 500 to 700 could be fit onto a pot at one time. "

I would say you are pretty close to being correct.  500 hd of 40-50# pigs is a load on a standard (no deck) pot.  700 hd would probably fit on a pot with a deck over the belly, a deck in the top and 3/4 doghouse.

The 1200+ hd on a load is with weaner pigs that weigh under 15#.