I have this RGB jpg georeferenced map image:
When I do unsupervised classification with 5 classes
It only gives 4 classes
The original image was generated from CS6 and is georeferenced.
Best bet to get the pixel count per class would be run the "Reclass" Tool on the original Image.
Choose the number of classes to be 5. You can manually choose the classes if you wish, although from the image shown you should be able to use the default settings.
Run the Tool and you should be able to Open an attribute table with the values of the Classes and the number of Pixels per class.
A second option, although untested, would be to the the "int()" function in the Raster Math Tool in Spatial Analyst. This will convert the Raster into a 5 classed integer raster.