Check your owner’s manual. I believe the brake you are talking about is referred to as the “parking brake,” and its primary purpose is to hold the car in place when it’s parked.
The term “emergency brake” went out of fashion when dual-diagonal braking systems came into being, which was a LONG time ago, and the parking brake is not designed to stop the car.
If you pulled hard enough (not easy to do) you could, eventually, stop the car with the parking brake, but it would not be easy, and I don’t recommend it.
It’s highly unlikely that you would lose all brake fluid at once. The primary braking system would still do a better job of stopping the car, even with low fluid, than the parking brake will ever do.
Use it when parking, but don’t expect it to stop the car in an emergency situation.