M does not mean million, M means mega which is ^6
so 6M would be 6,000,000
you're right in the sense that saying 6M would be 6 million, but M does not abbreviate for million.
also, saying "kk" is the same as saying "M" because they are both SI abbreviations. it's literally the same as if you said 1000k instead of 1M. k is kilo, as someone said before, so it's just ^3, or add three zeros.