However, some languages mess this up a bit, and I figure it might be of some interest to describe two examples.
The Normal Noun
If the noun phrase is any other case than nominative or accusative, the noun is in the singular and its expected case, while the number likewise is marked for that case. With the nominative or accusative, the noun itself is in the partitive case (which also is the case when the number is in the partitive), and the number is in the nominative form (or rather, numbers have identical nominatives and accusatives, except for 'one').
The Abnormal Noun
Some nouns lack singular forms, and can thus not abide by the rules laid forth above. Instead, the number adjusts, and is marked for the plural. This even goes for the number 'one', giving us monstrosities like
'yksissä häissä' - 'one-plur-inessive wedding-plur-inessive' - at one weddingbut also
yhde-t bilee-tThis is even more sick, as ordinals too get this treatment, giving us ugly monstrosities like
a party ("ones parties")
kolm-ans-i-ssa festare-i-ssaOf course, in Finnish each element of the numeral (except 'toista', roughly "-teen" as in thirteen and such) is inflected for the case of the NP, and each element of a numeral is also inflected for ordinality, etc.
at the third festival
Further, the comitative case lacks formally singular forms, and thus whenever that is used, the numeral also needs to be plural - even if that plural is one.
Russian has a peculiarity going on, whose origin is the defunct dual form. The dual was identical for some nouns in the nominative to the genitive singular (but not for all nouns, e.g. feminines had a distinct dual). This has generalized so almost all nouns, when following the numbers two, three and four, take the genitive (when the numeral is in the nominative, mind you!). With other cases, the noun and the numeral are in the same case and in the plural number.
With accusatives, inanimates behave like in the nominative example above. Animates, however, take the plural genitive from two onwards.
Certain numbers - thousand, million, billion - are really nouns, and the "real noun" is in the genitive plural.
In Biblical Hebrew (maybe in modern too; I don't know and will not try to find it out today - no diss of modern Hebrew, but Biblical just is so much more cool) the numbers three to ten take the opposite gender's congruence marker. Thus, 'five lads' would be five-fem.sg lad-masc.plur
There is also a 'construct'-number, which signifies 'n of', but has no gender congruence. These construct numerals can also take possessive suffixes for 'two of us' and the like.
Finally, in modern Hebrew, there is still a dual, but this is used only with:
- nouns that naturally occur in pairs, even for genuinely plural numbers of the noun, and with some pluralia tantum (that also naturally occur in pair-like structures, I guess?)
- units of time