Portsmouth have sold over 14,500 season tickets, out of their ground capacity of 18,931, so they say effectively they can't sell anymore season tickets, because they are under pressure to accomodate demand for match day purchases, in addition an additional 2,300 seats must be allocated to away supporters.
Tickets to the home supporters are now on sale for the first 4 home games, the Millers visit 3rd September 2017, the away supporters are in the Milton end of the ground.
So if the Millers take up their allocation, then Pompey fans season tickets, there are 2,131 tickets for match day purchases.
But if the away club sell below 1,300 tickets, Portsmouth can allocate some more tickets to home supporters.