2 can invade you at once, it's very frequent. You have to be anywhere other than Majula to be summoned by the ring. I wouldn't suggest waiting in the Bell Tower, as you'll be invaded almost as soon as you open the door 90% of the time.
It's not really popping up for me often and the only message about it I got was almost instantly shot down by challenging area boss.Does making myself more undead increase the chance of invasion? I can't remember.