As far as I understand it onions are self-compatible and they can pollinate themselves or cross pollinate with others.
Even though you might get some viable seeds from one plant, the chances that it's going to produce quality seeds worth replanting are kinda low. Usually a population of 50+ plants is necessary for healthy genetic crossing.
You could spend the few bucks on a seed pack next year to play it safe, but also plant some of your own saved seed just to see what happens.

Thanks, madroneb, that makes sense. The original plan was to leave 10-ish plants to over-winter and produce seeds, but the plants didn't get the memo and only one survived.