There was plenty of crew, 1435 of them were killed with only 1080 rescued.
Problem was a B-29 recon of Tokyo was seen by everyone in the city, they panicked and sailed Shinano to be completed in the Inland Sea without all the watertight doors fitted and unfinished bulkheads so water spread throughout the ship.
OK, that makes sense I just remember reading the story about it getting torpedoed and the sub crew couldn’t believe it sank.