2 by land, flight unlimited ...but even by air they will check on you after some excempts ... depends the officer if he believes you live and work in Thailand.
its 5% of your salary, ax 750 THB which is SSO - Social Security ... you can choose a hospital near u from the SSO list (if they have plave available , otherwise need to choose one more far away). Its covering 2 Mio THB per year.