Many languages have words expressing indefinite and fictitious numbers—inexact terms of indefinite size, used for comic effect, for exaggeration, as placeholder names, or when precision is unnecessary or undesirable. One technical term for such words is "non-numerical vague quantifier". Such words designed to indicate large quantities can be called "indefinite hyperbolic numerals".
Other specific numbers are occasionally used as indefinite as well. English does this with count nouns that refer to numbers: a dozen/dozens, a score/scores, a hundred/hundreds, and similarly thousand, million, billion. Unlike cardinal numbers, these can be pluralized, in which case they require of before the noun (millions of dollars, but five million dollars), and require the indefinite article "a" in the singular (a million letters (indefinite) but one million letters (definite)).
Umpteen, umteen or umpty is an unspecified but large number, used in a humorous fashion or to imply that it is not worth the effort to pin down the actual figure. Despite the -teen ending, which would seem to indicate that it lies between 12 and 20, umpteen can be much larger.