There were 1020 multiplayer battles fought in 2013, with an average of 9.5 players, out of 180 unique players.
(Slightly up from 1000 battles with avg 9.1 players in 2012.)

There were also 1562 battles fought by 150 SP players in the singleplayer campaign.
(Up from 950 SP battles in 2012.)
GCM MP battle totals by year:
2013: 1020
2012: 1000
2011: 823
2010: 128