The latest: 7.23.20, Release 7.23 has LTS. Meaning: Mendix will long-time-support it and solve bugs if need be. So bugfix-upgrades can appear (though less frequently over time) and you might want to have to implement a bug-fix-upgrade, which is no big deal.
Quite recently Mendix has added a very usefull tab “LTS/MTS Releases” to the Mendix Studio download pages: