No, it isn't normal to take so many visits --- but you ought to be grateful that your dentist is spending the time to make sure that the tooth will be OK in the end, and hope that the whole procedure is successful. It depends upon which tooth is being treated -- if it is a molar, it may have up to four canals to find, clean and file them out to the correct length, and to make sure that they are clean and infection free before inserting the root canal fillings themselves.