That imponderable requires an answer
A smartie is 8mm x 3.5mm and ovoidal in shape.
If you had a sqaure metre tray, you should be able to place them 125 x 125 in the bottom = 15,625 if arranged nearly
at 3.5mm in height, we are looking at approximately 280 smarties to reach 1 metre in height.
Assuming that our smarties are neatly stacked (but not perfectly optimised with a staggered layering) then 4,375,000 per cubic metre would seem reasonable.
If you however were thinking of a "pour fill" rather than a neatly stacked fill, then I would guess this would reduce the storage capacity a little - and for convenience of calculation, I would guess around 4,200,000, or 4.2 Megasmarties per cubic metre.
Old wembley stadium had a design capacity of 1,139,100 cubic metres and from googling, it seems the new one has a very rounded 4,000,000 cubic metres of space to take you to the brim.
So Old stadium = 4,784,220 Megasmarties ( 4,784,229,000,000 smarties)
New Statdium = 16,800,000 Megasmarties ( 16,800,000,000,000 smarties) - or 16.8 Terrasmarties
Not my answer, Google is your friend
..."My son," said the Norman Baron...."The Saxon is not like us Normans. His manners are not so polite. But he never means anything serious till he talks about justice and right"...