The total build was to provide for issue to units and reserve, and cover operational losses. At the end of March 1918, audit of losses due to the German offensive showed there should have been 203 4-inch mortars with units in France, 74 available as replacements in France, and 27 available from the February factory output at Home - clearly many mortars had been lost, destroyed or otherwise written off since introduction in 1915.
Using a figure of 200 front line mortars (June 1917 to November 1918) could imply 3,000+ rounds fired per crew, but even this is bit simplistic. The statistics of rounds fired for each of the four 4-inch mortar companies of 5th Special Bn, Royal Engineers, would be more meaningful; for example Foulkes describes No.1 Company with 40 mortars firing 2,300 rounds (thermit then gas) in 15 minutes at the beginning of a bombardment on 1 December 1917. So only 60 rounds per mortar from one company on that day, yet that could be regarded as a fairly substantial stonk.