Why not you trying to give some effort and come back with specific question
whatever if you are planning to give alert then its ok we have no problem just go and code with checking number of Passenger before Inserting new One.
You can count Number of Filled rows and can Compare to your Limited Seat which is 60.
I am sure your code will able to say "Oops All Seat are Filled Choose Other Days for Travel"
Navigate
MSDN-[Accessing Data with ASP.NET][
^]
Feel free to come back with Specific question and your Effort.