What does the quantity of treasures showed on the map means?
What does it means when the quantity of treasures reduce?
the number below each village represents the number of treasures currently held in that village. When that number declines, this means the player in question lost some treasures recently
The numbers you see on the map show how many treasures are in each active treasury. Each treasury generates Victory Points (which determine the winner of the server), crop for the village and border influence. These effects are limited by the size of the treasury building with maximum cap of 4000. Treasury village can hold more treasures than this but those additional ones don't give the benefits listed before. Generally speaking you could think of the treasure numbers as rough indicators of the strength of the kingdom that has them, the more the better. That being said that isn't always the case of course.
When someone loses said treasures it means one of 3 things:
Enemy attacked them, breached their defenses and stole some of them - this will also steal some of the VP if the attacker had lower VP than the defender
They built additional treasury space in their other villages and treasures always flow to fill the gaps
They had a friendly player attack them to redistribute the treasures to other dukes/king
The first one is obviously bad it means the defense failed and the village was breached, the other two are more good or neutral actions.