The Madrid-Barcelona "air shuttle" by Iberia has up to 26 flights a day in each direction https://www.iberia.com/es/news-updates/relaunched-the-air-sh... but they use more than one plane. Still, a lot of cycles! In the peak hours there can be a plane leaving each 15 min. You don't need to book a specific one, just a ticket for the shuttle service and then you just turn up at any time.
