The most common word for one in Russian is один in its various forms. Morphologically it is an adjective, which means it occurs in masculine, feminine, neuter, and plural forms in all six cases:

Since один is an adjective, it has to agree with its noun in gender:

Why would you need the plural of the number one? You may encounter the plural of один when specifying that you have one item that is pluralia tantum. Plurale tantum are nouns that only occur in the plural. In English we have a few nouns like that, such as pants. You don't say “Hand me the pant” even if you mean just one item of clothing. Instead you say “Hand me the pants.” In Russian the words брюки pants and часы “a watch” only occur in the plural, so if you want to specify one pair of pants or one watch, you theoretically can use the plural of один:

Alas, the Russian number system is not entirely stable, and occasionally you might find an odd bird who objects to using одни with these words. The issue of how to combine numbers with plurale tantum is quite complex, so for the moment trust me that the Russians often say it this way.