No to the first question, and Yes to the second. It is my understanding that each MRU encapsulates an external stimulus and the POV's reaction to it. Even small things within the environment will stimulate a realistic character and reveal more personality to the reader. These things need not be tied to the scene/sequel at hand (Meaning that they don't always have to move the plot.).
The MRU is a means of plotting, writing and editing complete motivation/reaction cycles from a character's POV. In structure, every reaction requires a motivation and every motivation requires a reaction. This gets down to minutiae in plotting and can make for a very long outline, but the method ensures that there is nothing superfluous written. In editing, the method ensures that nothing has been left out.
There must be at least one MRU for each phase of scene/sequel, because the plot of the story requires motivations and reactions. So there would be at least six MRUs per scene/sequel. Again, it is my understanding that there may be more MRUs per scene/sequel, but never fewer.
(For those reading the forum who might question the method, please try it instead. If critiques of your work include "Why did she do that?" or "This didn't make sense," it's almost guaranteed that something is amiss with motivation/reaction cycles.)