Sector marshals need to fill in a duty rota this year.
If the clerk of the course thinks that there is not enough cover for a session (practice or race) then he will delay or cancel the session.
I would have thought that delaying it until marshals are signed up on the spot will not be practicable so I can only see cancellation as a result of not enough marshals.
So if you want racing to go ahead then sign up.