Collective Numerals for People
Russian has a special set of "collective numerals" used to emphasize a group acting as a single, cohesive unit. The most common ones are двое (two), трое (three), and четверо (four).
When you use these numerals to count something, the noun that follows must be in the Genitive Plural case.
You will use them frequently to count groups of men or mixed-gender groups, especially when referring to tight-knit companions, roles, or general descriptors like guards, strangers, or buddies. (Note: they are not used for groups consisting only of women).
Counting Baby Animals
Collective numerals are also traditionally used when counting baby animals, such as puppies and kittens. Remember to put the animal noun in the Genitive Plural (щенков, котят).
Plural-Only Nouns
Some nouns in Russian only exist in the plural form, such as ворота (gates), часы (watches/clocks), or сутки (a 24-hour period/days). Because these words do not have a singular form, you cannot use standard numbers like два or три with them. Instead, you must use collective numerals (двое, трое, четверо) to count them.