https://www.southwest.com/help/changes-and-cancellations/ove...

https://www.jetblue.com/legal/notices-to-customers

However, this isn't a guarantee you won't get bumped. This just means that the maximum number of tickets sold to customers equals the number of seats on the plane. If the plane changes, so do the number of available seats. And not all passengers are customers. Some may be crew deadheading to another location.

More generally, it is less common for low cost carriers to overbook flights because their passengers tend to be cost-conscious vacationers who paid for the flight out of their own pocket and aren't about to forfeit the fare. Usually these passengers show up at a very high rate. So there usually isn't even a reason to want to overbook these flights.