Yes, you can fill the form of any number of courses in Delhi University. But, for BBA, if you have to fill the form for DU JAT Entrance Exam. You can apply for DU JAT on DU Undergraduate portal. You will have to pay a fee. But do it as soon as possible because tomorrow the online registration portal will be closing. After that, you can not change or add courses you want to pursue in DU.

Yes you can dill forna for test based admissions though you have filled merit based form.  You can apply to any enterance and any number of enterances.