I saw online somewhere this word declined singular and plural. How can this word have a plural form?
It can be plural when used with a noun which lacks singular forms or which refers to a collection of things, e.g., unae aedes; unae litterae. Also, when used with another number, e.g., similitudines, unae rerum, alterae verborum. Also, when used with the meaning ‘single’ or ‘only’, e.g., uni sex dies; tres uni passus.
It’s worth reading Lewis and Short’s entry for unus: http://athirdway.com/glossa/?s=unus