It was originally 2800 EMS (5600 without multiplayer), but with Extended Cut it's apparently easier to get. Easiest way is to play multiplayer and just collect the stuff lying around the galaxy, pick up sidequest items and turn them in to Update certain Assets. Regardless of playthrough, you should be able to get enough by simply 100%-ing the game. RShepard227 (talk)
This page details the various endings based on your EMS - 2800 or more SHOULD allow the synthesis ending, although there are other factors.