If previous cycles are followed, 5.8 is likely to be the final series 5 release so 5.6 to 5.8 will be the last in-series upgrade.
Linux Lite series 5 will get updates until end of support in April 2025.
I have no information about LL series 6 but previous cycles imply that 6.0 is likely to be released in June 2022. If the release pattern is followed, moving to LL 6.0 will require a fresh install of the OS followed by re-installation of any extra applications and copying personal data across.
Hope this helps.