I don't know when the next one will happen, but they will advertise it on
https://swa.pilotcredentials.com
Just keep an eye on that site, and make plans to attend Women in Aviation and OBAP if you can swing it. I'm sure recruiters will be at both of those events (usually March and August I think).
There's been a lot of numbers thrown around, but last I heard was 750+ in 2018.
Apply to every window, update your app regularly, and attend as many recruiting events as you can. They want to see that you want to work at SW.
Good luck!