In sports, having a deep bench means having a large number of very talented players. As not all players are playing at the same time, very talented players will be sitting "on the bench" waiting to play. For startups, this means having a very smart, hardworking team and not relying on the founder to be the only star of the company.
Example: "It is easier to get funded if you have a deep bench. No one wants to think that the fate of the entire company rests on one person alone."