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.
Sources:
Chappell, Mike (2000). British Infantry Equipments 1908-2000. Osprey Publishing. ISBN 1-85532-839-9
Skennerton, Ian (1988). British Small Arms of World War 2. Margate QLD (Australia): Ian Skennerton. ISBN 0-949749-09-5.
Light reading:
Issued or through their career? And for a rifleman or machine gunner?