Nos. is a written short form of “numbers”, used when you mean more than one number. You put it before the figures, the way you put No. before a single one: “rooms nos. 12–14” means “room numbers 12 to 14”. People do not really say “nos.” out loud; in speech they just say “numbers”. You will meet it on lists, forms, catalogues, invoices, and older notices. Some writers capitalise it (Nos.) and some do not (nos.); it is the same shortcut. It feels a little formal and a little old-fashioned. In everyday mail many people now write “numbers”, “#”, or just the figures. It is only a spelling convenience, not a new word with a new meaning.