Only 2 visa exempt entries by land per year that's the only rule.
There is no 6 month rule, that was cancelled years ago.
In theory its unlimited by air visa exempt or with visa and by land with a VISA even tourist visas. But its down to the immigration officers decretion.
If they think you are living here without the appropriate visa they will refuse, but it should not a problem with your record.
If you are doing a out and back visa run by land go with a company.