"On" in French could be translated as "one" but not the number; it's like "one" as in "one never knows what may happen next." In English this has an old-fashioned ring, but in French it is the norm. It's a singular form (3rd person singular, like "il" (he) or "elle" (she.)

 Officially, it's singular. The verb is always singular. You always say *on est* ; *on sont* would be incomprehensible.
But since *on* is more and more replacing *nous* in spoken French, you often find now an inconsistent usage: *On est allés* (we went) with a singular verb but a plural participle. The correct form should be *on est allé*, but many good writers and journalists use the plural in the participle if the meaning is "we" and not "one" or "they". In a sentence like *on a raison de dire*, *on* means "people", "they", "one" : "people are right in saying", "it is right to say". But when after a soccer match you here supporters chanting *On a gagné*, it definitely means "we won".