Wikipedia puts their current fleet at 745 planes[1], and I'm not seeing more than 200 Southwest planes out there by my quick count. Could be close to a third of their fleet, which is still bonkers to think about.
LionAir, which had one of the 737 MAX accidents, has a similar story, and will probably end up being a similar size. Only started in 1999, most of the new aircraft model launches are done with them, and they have 234 planes on order.
[1] https://en.wikipedia.org/wiki/Southwest_Airlines_fleet#Curre...