Nifty and Sensex are the benchmark index for the companies traded on their respective stock exchanges. Nifty is the benchmark index of National Stock Exchange (NSE) and Sensex is of Bombay Stock Exchange (BSE). The number used after Nifty and Sensex represents the number of companies that index represents.
Example: Nifty 50 represents the average of 50 top companies from different sectors listed on NSE. Similarly Sensex 30 represents the average of top 30 companies listed on BSE.