Compare to "one of the founders".
Does it depend on the specific person you're talking about? Google has 7M+ results for the phrase but I can't search it on NGrams successfully because of the hyphen. (I think.)
Learn English – Is “one of the co-founders” ever correct
word-usage
Best Answer
Theoretically, you could be referring to people who could be either "founders of a company" or "founders of companies". In the latter case, the founders could have founded their companies independently, so "co-founders" would be used to indicate that they did not in fact found separately. I think "co-founders" is redundant in virtually all other uses, though.