You can add a sequential number as a custom row number of a row using SET clause and increment at next row.
Suppose users table has columns name and age.
When you want to show the result you want to add extra column serial_no.
So using below query you can achieve custom row number:
SET @serial_number = 0; SELECT (@serial_number:=@serial_number + 1) AS serial, name, age FROM users LIMIT 0,50;