How many novocaine shots do they use for root canal?

Few of them(3-5) They have to be sure that area is numb so they could work on it.
Depends. Sometimes with an infected tooth, the shots may not be 100 % effective so many shots may be required. However, not too many shots are required for a root canal.
As many as necessary. Prior to starting a root canal, a dentist wants the tooth to be completely numb. Different teeth require different amounts of local anesthetic ("novacaine") in order to become numb...And different people will require differing amounts. In general, though, the dentist will use the lowest amount possible that achieves the required level of "numbness".