Hmm... I don't know if the 3 mechwarriors they give you at the start count towards the achievement, but I guess not. So I had at least 9 pilots in total at some point (considering the dead ones too). I remember now I was trying to hire only pilots with unique portraits (just cause they seemed cooler), given they were rarer I guess that's why I had fewer pilots at the end. But when I finished the campaign I lost interest in the game.
I won't say I loved it, but I enjoyed it, I found it a bit slow sometimes, but it's a solid turn based game nonetheless, plus I immediately liked Lady Arano and the whole Restoration story, that's what really pushed me till the end of the campaign.