I think the target is 500k, but I believe the capacities are higher than this. Sure somebody could wizz through the list and work it out!
Capacity for all the matches is approximately 600k (double header matches only included once).
500k definitely looks achieveable pretty much comes down to the the matches at at the MS, Wembley and Old Trafford (accounts for 40% of the capcity) . Given Wembley and Old Trafford are the biggest matches (those two dates account for the majority of the tickets so far sold) there is a good liklihood of strong crowds there.