In addition to your N7 score (the number of multiplayer characters you have "promoted" there is your galactic readiness rating. The default for this is 50%, which means that, of the score you accumulated, you can actually use 50%. Completing Multiplayer missions raises that rating. The score drops if you don't play multiplayer for awhile, but never below 50%.
As I said above, you can get the best ending if you do all the side quests even if you never touch multiplayer.