see Benjamins answer. Essentially a non imm o is for 3 months and then an in thailand extension (yearly in my case as I live here). A re entry pass is obtained on each exit from thailand (I only visit family in uk once a year) so single. A multientry pass allowing 3 reentries is an alternative option. These are also obtained in thailand prior to leaving and return.
I have had both non imm oa and o extension based on retirement ...overall, particularly if you already have 800k in a thai account I found the o retirement extension option both easier to get and overall cheaper due to less certificates and insurances. Particularly as you pass age 65 and insurance premiums rise steeply often.
currently most land borders are closed or restricted. Neighbouring countries are opening up a little but the 'almost same day' border bounces are still not possible I think ...paperwork for entry and exit and re entry to thailand requirements blabla
Tod is right, altho I often take insurance for the expected length of stay and even higher coverage ...cant be bothered with multiple policies to cover medical eventualities.