How much stainblock paint to use in bedroom and where?
There is no problem with multiple coats of stain blocking paint sealers.
When I use these I coat the entire wall, just sealing a small area can create the look of a stain in that area when the covering color paint is used.
I use zinsser and kilzs fairly often and I am sure any other shellac type paint or stain blocker would or should be applied or need the same (coating the entire wall where used).