If you've got SRAM brakes, then you've got to push/pull the syringes to cycle the fluid through & then pressurize, so if you've got old contaminated fluid to start with, then that has to purged out first, so you'll end up using more than if you've got Shimano brakes.
If they are Shimano, then assuming no issues, then you should be able to fill at least 4 brakes from 100ml.