So, if you are filling an application form and if it is asking for CBSE registration number, then your CBSE HSC roll no. is your registration number. In the column of registration number, you have to fill up your CBSE 12th roll number. And in the column of number of appearances, since you have qualified your exams in one go i.e., without compartment, then you have to fill 1 there.

For more information, you may visit the instructions and the brochure of that particular exam/ institution whose form you are filling.