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.
To the person who removed my comment, if he has divorced since his last extension then it will matter. He will need to provide the papers to show he has custody.
He doesn't say he he divorced before his last extension or not.