The max number of instruments one can add in the watchlist is 100. Usually, watchlist are created for the stocks that a trader wants to keep his/her eye on and want to take the trading decision.
If you want to track the constituent stocks of any Index, then we have alternatives. Like for example, you want to see all the Nifty 500 stocks in a single view. Simply search Nifty500 > Go to In-Depth. You will get the list of all the 500 stocks with LTP and more details.
If you are using tv.dhan.co, select the Indices tab from the top menu, a dialog box with all the major index and Underlying F&O stocks will open. Select the desired index and you are good to go.