When number of columns are less then 15 i am able to define cloumns in destination text file but when number of columns in table is more then 15 task is not able to generate columns in destination text file.
You should have not problem to create text files with more than 15 columns.
Maybe if you create a dts pkg with 15 columns the first time and you want to include more columns later you probably wont be able to do it until you inlcude this columns in source part under transformation task.