Hello Haseena,

I would like to tell you that you have to give the exam with the already registered number as the registration for AIIMS started at November and was going on till January 14th.

I would suggest you to visit the careers360's link given  below which has a detailed explanation of the AIIMS 2019 Important Dates so that you can get an idea of all the dates.

The link for the same is:https://medicine.careers360.com/articles/aiims-mbbs-important-dates

Hope this helps and all the very best!