In reality, many people would not pronounce the th in the plural form months.  It would sound like one munth, two muns.  But be assured that there are also people who do pronounce the th in the plural form.  But then you have to go mm-uuu-nn-th-sss.

Only pronunciation difference is that 'months' and 'month's' have an 's' and 'month' doesn't. Other than that, they're said the same way.

Thank you :)) it helped me a lot.