Originally Posted by chicken10der
'Master and Commander' has no defined number yet (why it's not in the achievement description itself is beyond me), but it is based on the Effective Military Strength (total x readiness). The number seems to be somewhere around 3500-4000, and the achievement will unlock when you press A to land at the Illusive Man's base. I ended with 3328 after doing every mission and planet scan and didn't get it. There is one report of someone getting it without playing any MP (thus a readiness of 50%) and that was only with never making any 'kill' decisions/actions throughout all 3 games, including DLC for all 3 games as well (for example, not killing the villain from the ME1 'Bring Down the Sky' DLC is the only way to get the Batarian fleet as a war asset in ME3, and you have to save the Rachni queen in ME1 to get their full war asset as well)
Strangely, if you don't play BDtS, you can still get the Batarian fleet.
Still, I haven't seen people with more than ~3500 EMS at the end of the game. I had 3503 and didn't get the achievement until I played some multiplayer.