British soldiers in WW1 wore 1908 pattern webbing instead of the usual leather belts used at the time, am invention by the American Mills Equipment Company. This webbing had 2 ammo pouches, each containing 75 rounds, so the answer to your question is 150.
Remember that the weapon on the front given to the common man was the Lee-Enfield Mk III, which a trained user could use to take 20-30 aimed shots a minute, so 150 rounds might not last long.
