So we have one actual backdoor attempt, one OpenSSL bug that may have been a backdoor attempt but was more likely an honest bug, one tongue-in-cheek attempt men

So we have one actual backdoor attempt, one OpenSSL bug that may have been a backdoor attempt but was more likely an honest bug, one tongue-in-cheek attempt mentioned by Linus, and that same attempt repeated seriously by his father. Assuming Linus was being serious and not facetious, I would say that's one attempt. And if the NSA really wanted Linus to backdoor his kernel, I imagine they would have approached him in-person and asked (as they have been known to do), so I'm not convinced that the two other attempts were them either.