The answers are: most dreams, yes and yes (I think)
Dream signs don't have to appear in every dream but they generally crop up a lot. For example the closest thing to a dream sign of mine is being at school, which seems to happen in around about 1/3 to 1/4 of dreams.
I am sure mantras can help with recognizing the dream signs, as they work well with other lucid dreaming aspects.
And there's no reason I know of stopping people from having multiple. I don't myself but I'm sure some people do.
Good luck!