I pictured him/her making a completely 3-D realistic helmet sitting upright with faceguard and not knowing how big to make the helmet's dimensions, so I said to monitor the batter to AT LEAST 50 (ie more than fifty, not batter for fifty) then you'll have a frame of reference as to whether it's about the right amount of cake.
I know what you meant by your first reply, but I didn't know if he was using some funky 3-D half sphere pans that he didn't know servings for, etc., not just sheet cakes that might dictate an obvious amount of servings, to me it's not pastry blasphemy to use the batter as a frame of reference to give you some idea, if you're aware of how it's going to be put together and how small your waste would be. I would think this shape could be accomplished with very little waste. I pictured a bowl/half sphere cake, on top of round cakes. Where you cut the front flat, you could use the "moon" shape leftover to build out the faceguard part. You'd sculpt a little to taper it inward at the base. Whatever...