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.

Vote against.
4koma is a well-established format; 5koma evolved as a darker interpretation of it.

3koma IMHO shouldn't exist; same for your proposed 6koma.

post #1013222
Want to tag that 8koma too?

i'm obviously against this tag.

regarding 3koma, it has more entries than 5koma. so at least in danbooru, we could say, it's pretty much established too. and users are active in using it. so for me at least, it has reason to stay.

regarding 6koma, i tried searching pixiv with 六コマ 6コマ and there are results actually.

but the question is, to what limit are we going to specify the number of panels?

particularly the tall_image comic? post #1003121 for example has 20 panels.

similar to the multiple girls/boys tags, we could do 6+koma. anything with 6 or more panels gets in.

My take is that xkoma should be defined slightly more tightly.

The bulk of a comic should be of a specific xkoma for it's pages (where appropriate) to be tagged as such.

Otherwise if it is a single image or a random smattering of variously numbers of panels per page it should just be tagged comic.

So the tag would only be necessary if there was actual comics of 5koma pages or 3koma pages or any variation there of.

Beyond that as zekana said treat it like boys/girls.

Who the hell is going to search for this proposed 6+koma? IMO, 4/5komas are interesting because they manage to make a joke in a limited amount of "time" but if you increase the amount of panels then there ceases to be a limit, and it becomes just a regular comic.