Following the pattern of 3koma, 4koma and 5koma, a tag 6koma certainly could be used if people wanted it. (I don't know if there are many 6koma posts around here, though.)
post #732561 would be tagged 6koma; it's the only example I know of at this moment.
EDIT:
After reading the thoughts below, I agree with the idea of using 6+koma for what could otherwise be tagged 6koma, 7koma, 8koma, etc.
Updated by jjj14