how many swaps or pledge parties are there per wee

First of all, pledge parties are illegal. Every sorority tells their pledges to not attend off-campus pledge parties whatsoever.
For the first semester there is one swap roughly every week. Every once in a while there will be weeks were you won't have scheduled swaps. And sororities usually schedule date parties every 2-3 weeks. Some weeks you'll have a date party and a swap, some weeks just a swap, some weeks just a date party, and some weeks nothing for the first semester. Also, sometimes swaps get cancelled last minute.
For second semester, swap season is basically over. Sororities will have a swap maybe once a month, and a few date parties throughout the semester but second semester is known as darty season and bar season.

Is that true about pledge parties being off limits? That's what I was looking most forward to!

They're not condoned but they happen. I hosted a bunch for our freshman this year. And yes not all the girls go but it seems like more of a thing now like don't ask don't tell. They were taboo when I was a freshman too but we still had them when possible