What SpaceCommander says is correct. Capturing your opponent will not count for quests, nor does it count for credits. It only counts for winning that round. To find out how many ghosts you captured, you can divide your credits (aside from your win bonus) by 5 and that will tell you.
To try to get more ghosts:
1) Try to make sure you play all 3 rounds
2) Play with more people on the other team, you get more ghosts that way