Capacity is always stated using 2¾" shells unless specifically indicated. The 2¾" star crimped shell is the industry standard for 12 and 20 gauge capacity. Please note​ that certain shells that are roll crimped (plastic disc over the shot and an inverse turtle neck edge) may decrease that capacity in some guns as they are slightly longer overall.

Per Mossberg, their capacity is counted using 2.75" shells and counting the number in the tube plus one in the chamber. In this case, this would be eight 2.75" shells in the tube and one in the chamber for a total capacity of 9.