There are several ways of doing this:

r.stats or r.report by using Grass GIS.  

Count Raster Cells from the plugin “LecoS - Landscape Ecology Statistics / Calculate metric”

Molusce (plugin) / Area Changes / Class Statistic 

Histogram

I am certainly going to forget to mention some...

Thank you so much Hernandez...

Open your classified raster in ERDAS IMAGINE and view attributes (viewer > raster > attributes). You can get no. of pixels under Histogram field.

You could use the qgis_plugin Raster Pixel Count By ClassBreak . It is quite simple, you can define your breaks and it will give you standart stats (mean, sdev, max, min...) plus the count of pixels by break and the corresponding area in a pop up window from which you can copy the results you are interested. See figure as an example.

Another alternative is using QGIS Geoalgorithms\ Raster layer histogram, just define number of bins and it will create the histogram graph and a table added to the Layers Panel that you can just copy to further processing.

If you look for the corresponding area, then Raster Pixel Count By ClassBreak will provided it in a single step as suggested by @Inti Luna .

I have the latest version of Qgis (Madeira 3.4.2) but I can not find the plugin Raster Pixel Count.... Somebody know why this is?

Hi Arun Prasad Kumar , I would like to share a simple method that has not been mentioned in earlier comments. In QGIS, get the 'Semi-automatic Classification Plugin (SCP)'. You can get the pixel sum for each of your 14 classes via SCP > Post-processing > Classification report. Input data will be the classified raster itself. Additionally, you will get the percentage that each of your 14 classes make up across the whole region of interest, e.g. class 1 (20%), class 2 (2%) ... , summing to a 100%.