How come there is adjective and adverb in this table? Normally there is adjective (which really is verb in some languages) and noun in adjective tables.

I wasn't thinking enough, I guess... I'll change the second column to "certainty."

I guess there are some cases where adverbs can be used, such as "often" and "seldom". Maybe there's no corresponding noun to them either (oftenness?)...

Yes, this is one of the limitations of using English as the base language. In the case above, though, I think "often" is the same as "frequent," and actually is related conceptually to "many," isn't it? Also, I guess "seldom" is conceptually related to "few." But it's all very complicated. We're sort of going through the same process that Roget went through when developing his thesaurus.