Maximum amount of war assets from "promoting" level 20 multiplayer characters
Solution 1:
There is no limit of how many times you can promote (whether its the same class over and over or different classes). You will gain 75 points of war assets each time and a boost to your N7 ranking.
The only draw back of doing so is you will (very slowly) lock yourself out of seeing any ending other than the best ending if you get some ridiculous amounts of war assets.