For me it was 10. You get a 100 point bonus every consecutive match (100, then 200, etc), the maximum (i.e. it won't go any higher) is 1000. It's a little buggy though as some people have reported more or less than this before the achievement unlocked. Just
keep going and it'll unlock eventually.
This one is not as hard as it sounds -- I stink playing against human players but still managed to get this in about an hour.
Just go into a social versus match and play until the achievement pops. Do not exit, do not even go to a menu, as these will reset the consecutive match bonus. After each match (i.e. map change), you'll see your stats -- watch the consecutive match
bonus as it will go up 100 points per match, 1000 gets the achievement.
I would recommend Team Death Match, especially if you still need the
Welcome to Versus achievement.