I didn't realize there was a limit and have reinstalled macOS several times since creating the certs without backing up my private key file(s). I've asked to delete or add a few additional certs to my account but it's been 4+ weeks and they are still "working on it". What a mess. I no longer can sign apps and am still paying $99 a year for that privilege. Seems like some sort of warning or message to backup private keychain entries somewhere might have been nice (or a warning when creating the last one I had available)...