About Datafile Limitation In Oracle Data

i have database which contain a default datafiles(20). my database is running in archive logmode.i want to add some more datafiles,i have a adequate space on my server.I have already change a parameter dbfile=20 to 25 in init.ora.but still i couldn’t add data file. when I try to add datafile i got an error that u cann’t add datafile more than limitation.