This achievement IS retroactive with Title Update #8, so if you've acquired 50 or more Titan Insignias prior to the update, the achievement will unlock upon starting the game up.
If you have not obtained 50 Titan Insignias, progress for the achievement will carry over from before the update was applied. To earn Titan Insignias, you can either fulfill challenges tied to Insignias (such as playing every game mode on DLC maps) or you can purchase some from the Black Market. If you've played the game for quite awhile like I have, you will more than likely have a great majority of the Titan Insignias already.
With this new title update came more Titan Insignias to unlock, so now there's even more possibilities to unlocking them! These new insignias are tied in to earning stars in the new game mode, Frontier Defense.