That seems quite a lot of shafts. There were quite a few on the East London Line, to which I and others referred previously.
Were there any criteria about when to have ventilation shafts in a tunnel? I was reading recently about the problems the LNER had with its Woodhead freight services and the notorious Silkstone tunnel on the heavily graded freight line from Worsborough to Silkstone. The tunnel was fairly short at just under 300 yards, and, so far as I can tell on a 25" map, no ventilation shafts, although it was heavily used by coal traffic, and always had smoke problems which seriously affected the crews of locos, especially those on banking duties (including 'The Garrett' U1). Perhaps it wasn't deemed economic to dig a shaft down 50' or so (judging by contours on OS maps) to give better ventilation, or to open out the tunnel into a cutting.