The word "unit" means one. Darnay describes his fate as hopeless because no one person can help him. He realizes that the tribunal has fully condemned him. This is ironic because one person does save him—Sydney Carton.
Darnay has been sentenced to death. He realizes that millions of other people are in the same fate. Just as no one can save them, he thinks no one person can save him.
He had fully comprehended that no personal influence could possibly save him, that he was virtually sentenced by the millions, and that units could avail him nothing. (Book 3, ch 13, p. 222)
Darnay is sitting awaiting execution feeling sorry for himself. He does not make any actions to save himself, because he can think of noe. At the same time, his "hold on life was strong, and it was very, very hard, to loosen" (Book 3, ch 13, p. 222) because he does not want to die.
This contrasts strongly with Carton, who also values his life but values his love for Lucie more. Carton is willing to sacrifice himself for Darnay, because there is nothing else to do. They are up against impossible odds.
The revolution has grabbed a hold of the entire family and spiraled them out of control. Unless Carton steps in, nothing can be done. Darnay does not realize this though. He does not know that one person is actually going to save him.