spring?

It is not decided until right before spring rush. It depends on chapter totals at the time. Right now chapters are full but there may be some drops or graduations and that may free some spots up. Some years there is no spring rush but usually there are a few houses, usually bottom/middle tier, but there are few spots per chapter, like maybe 10 spots.

If you suicided or dropped then I don't think you can rush spring but maybe someone else can correct me.

There are usually a few hundred that rush for only a few spots.

Fall is the best chance. This is why so many on here encourage sticking with your bids and drop before initiation if you are really unhappy. Then rush again next fall after you have hopefully gotten to know active sisters that will want you.