It's accepted fact that pictures move, and that portraits talk.
However, why is it that the subject of a portrait can be in only one frame at a time?
Also, what happens, if a new portrait is made? As soon as it is complete, does the subject disappear (to maintain that there is only one subject across all the frames)?
On an extra(and possibly useless) note, I remember that Harry once got a Chocolate Frog card of Dumbledore, and Dumbledore shortly walked out of it. Ron commented something along the lines of, "Can't expect him to hang around all the time." So, do Chocolate Frogs also share their subjects with portaits? I understand that photographs, atleast, seem exempt from this logic. No idea about celebrity posters(like those of the Cannons and Krum) either.
Lastly, what is the point of having multiple portraits if they are almost always empty? Doesn't it defeat the point of having a magical portrait? I would find it offensive if I went to the trouble of having a portrait put up, only for the subject to promptly walk out.
The following questions touch upon a similar topic, but focus on the magic behind the sentience of the subjects, rather than their availability: