This authority gives a total stowage of 85 6-pdr rounds for both the Mk III and the Mk IV Churchill. http://www.wwiiequipment.com/index....t&view=article&id=66:churchill-infantry-tank- Other online links say 84 rounds.
Chamberlain and Ellis says 84 6-pdr rounds, as do a couple more reference books. There are stowage sketches here; The Churchill Tank Figure K - 39 Figure L - 17 + 3? Figure O - 21 + 4 That would make 84. Gary