All Boris needs is just one EU member state to veto another extension.
Also, though he is supposedly required to ask for another extension, he can do so under certain conditions - namely that the EU drop any backstop nonsense and drop any claims to judicial authority over Britain. Obviously they won’t accept that. In which case Boris can say “oh well. I asked for an extension. They just wouldn’t play ball”.
The Benn act which mandates he ask for an extension, also specifies the exact text of the request he is supposed to send.