If my algo goes to live trading, it will fetch the live data for calculating. Is this live data source the broker (like IB)? Or the Quantconnect since we subscribe the  live trading? 

If the data source is IB, it has an live data limitation about 100 instruments when subscribing the live data streamming. If you want to extend this limit, further fees will be charged. That would be a problem if the strategy is based on large universe.