You need to concatinate the two strings
1. Get the constant string eg. ABC
2. Retrive the total count value from the database (eg. count = 5)
If you are using the SQL server as your database.
You can also use file to store the data.
Now add two string
suppose string str = ABC
and int value = count + 1;
now,
string id = str + value.ToString();
Remember, you can only concatinate strings.