I heard that promoting a character in ME3 multiplayer permanently increases your minimum readiness value. Is that true? By how much? Can you promote enough characters that you stay at 100% forever?
While most actions in multiplayer increase your Readiness Rating from the base 50%, promoting a character in multiplayer adds 75 to your War Assets, permanently. You don't gain permanent readiness rating from this action, however.
When you multiply your war assets by your readiness rating, you end up with your effective military strength, which is really the number that matters when the game determines which variant of the endings will be available.
I think you're confusing Galactic Readiness with War Assets. Promotion permanently adds 75 War Assets. Having said that, you would have to promote over 30 characters before you could receive the 4000 EMS needed to get a good ending, while also only doing the Priority missions.