I'm trying to obtain values of area covered by dense and sparse vegetation through NDVI classification. I'm using QGIS (2.18), ERDAS 2014 and ArcMap.
It depends on what are you trying to achieve. The categories such as
sparse are subject to your understanding of that area or some previous research or study conducted.
NDVI calculation provides output in range of
+1 barren to greener. Each pixel in the image would be in that range. If you happen to know the intervals of the classes you mentioned in this range or take the help of this, you can classify the images accordingly, there are tons of documentation available on how to do that.