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.
"As of 2018, Southwest carries the most domestic passengers of any United States airline."
https://en.wikipedia.org/wiki/Southwest_Airlines
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.