Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
quantity [ˈkwɔntɪtɪ] n коли́чество; (large amount)
in quantity в большо́м коли́честве
an unknown quantity зага́дка
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.