In the set you get 25 pieces of brass shells.
On the photo above you can see a close-up shot showing a comparison between Mission Models .50 cal shell
and new 20mm shell. As you can see new shells are noticeably bigger - just as they are in real life. Under shells is the edge of metal ruler - distance between black lines is 1 mm.
On next two photos you can see details of both ends of spent shell - just like with Mission's earlier .50 cal shells, those new shells are also hollowed from one end to the depth of about 1/3 to 1/2 mm to create illusion of empty shell.
I was not able to find any photos of real 20mm Flak ammunition online, so I can't comment on accuracy of those shells. But during my visit to Mission Models, Jon Tamkin showed me a real 20mm shell he used as a prototype for those small ones, so I have no reason to doubt that they are indeed exact scale replicas. Unlike .50 cal ammo sets, which are interesting for both WW2 and modern figure and armor modelers, this product is addressed mainly to WW2 modelers. But 20mm guns are used on many modern vehicles and I'm quite sure the shells of their ammunition are similar enough to the ones used on WW2 German guns, to make those tiny 1/35 scale shells useful also for modern armor modelers. Some vehicles that immediately come to my mind are Bundeswehr Marder and Wiesel vehicles - both equipped with 20mm guns. I think the design of German 20mm ammo shells has not changed all that much since the WW2, so those 1/35 scale shells would fit perfectly on a diorama with one of mentioned modern German vehicles.
The price of this new set is the same as for older .50 cal sets: $16 for set. Once again this translates to 64 cents for one shell, but this time you only get shells - no ammo belts as none were used with this ammunition. So this set is not cheap, but I still highly recommend it for all superdetailers who can afford it!
Many thanks to Jon Tamkin of Mission Models for a review sample!