The number displayed on the Resource Map is a graphic glitch. There are 2 possible explanations, the later being the most likely since a different number is showing, for different people.
As you can see in the image below, the numbers are only there, so the game knows where to place land and where to place water.
The image is taken from
C:\Civilization 2 - Test of Time\Original\Terrain2.bmp
Also the thread you spoke of in the comments to your question may be this one: CivFanatics
In the thread it is suggested that the number originates from
C:\Civilization 2 - Test of Time\Original\Terrain4.bmp
Where a number is placed on each River Mouth sprite.