"Numbuh" is an intentional misspelling of the word "Number" that is used as the standard numerical designation for KND operatives. Throughout the series, the word "Number/Numbuh" is constantly spoken, usually without a clear "r" sound but without an emphasis on the lack of an r, as it often comes across in real life due to people's tendency to passively leave certain sounds out of words. Sometimes, it is clearly pronounced with an "r" sound, while other times, the sound is clearly and deliberately dropped.
Many characters usually pronounce the word either one way or the other. Numbuh 362, for example, is known to almost always pronounce the word with an "r" sound.
The title for Operation: N.O.-P.O.W.U.H., also replaced the "ER" with "UH".