Maximum of one on the roof, and then probably only on an AA mount. Although this was not common (well i've never seen one or even heard of one...) as the cromwell carried two BESA mgs in a hull and a turret mount I really could not begin to imagine the logic or practicality of such a mounting.....
Can I also ask why you are asking so many questions about MG mounts? Personally I think it is brilliant that you have the confidence to ask so many questions but as I said before, there is a certain pattern in your questions......
29 Topics started in one month?