Canonical information on how extensively the Imperium uses coldsleep / low passage for it's Naval crews

Are there any canonical (preferrably) sources on how much use the Imperium makes of coldsleep when crewing it's ships? I understand it is used to keep replacement crew on hand for occasions when casualties occur but that just raises the fascinating question of how that interacts with the chain of command. Do they keep spare captains on board? Does the spare captain get thawed out before or after the regular captain and the executive officer are dead/incapacitate?