Certainly it will depend on who you bank with, but JPMorganChase has 250,000 employees[1]. If even 1% of them are customer service representatives, bank tellers, or in other positions with direct access to your account (which I hope we can agree is an underestimate), that's 2,500 people right there.

[1]: https://www.google.com/search?q=chase+employees

And they all have access to all accounts? I would imagine only a bank teller from the appropriate branch would have access to this branch's accounts.

Maybe back in the 1960s it was like that. Doesn't matter which branch I open an account at. I can walk into any branch and the teller should be able to pull up my info.

Does it matter? There are also millions of people who could murder you or burn down your house or kidnap you and force you to withdraw your money from the bank. But just like a bank worker stealing money, these are all serious crimes and easy to get caught so we're pretty safe.