My parents having pension, I am lone son, I want them as my dependent, how much is their present income from all source limit to show them as my dependent in india

This is a moot point unless your parents are living in the United States. You CANNOT claim them as dependents if they are living in India.

If they reside permanently in the United States, then ALL world-wide income must be considered to determine if they must file a U.S. tax return and to determine if their income is low enough for you to claim them as dependents.