I did the same mistake, but it’s actually 3 on gdev: (see Se rendre compte stupidement que sur la gdev il suffit de 3 certifications pour être membre).
The certs only need to be active, the issuers do not need to be member. See Trouvailles d'exploration de données - #7 by HugoTrentesaux for an example in Ğ1 history where an identity became member of the wot with none of its certifiers member anymore.