Asp.net - Check The Class Has 10 People ? If It Does Prompt Or Save The Data To Database

i am using vb.net with asp.nethow am i going to check that this class has 10 people already if i continue adding the 11 one it should prompt me that the class is full once i click save button, i know is something like validating but not sure howi can first add a few student to class 2A and later on continue with adding to another class however, when i return to add more student into 2A and save, it should help to check like what i mention above i have this table call listOfStudent

listID class student registerNo
1 2A A 123
2 2A B 456
3 2B C 789

i have a aspx web page, i have a drop down list where i select the class

[code]...

when enter finish the first row of student data, user can click on the addNewRowButton to add a second row of student data and so on, then after all is done when the user click saveAllStudentDatabutton, it should check whether this class i selected from drop down list has already has 10 student if it does it prompt if it don't it will add in