Hey,

Let me guess that you are asking about JEE Mains 2021. So, let me tell you that the candidates can apply for all the 4 sessions at a tine, but you have to pay the application fee only for the number of sessions you want to appear.

You can also select only february session exam, and for the further 3 sessions, the application window will open briefly. So, you can fill the application forms then for those sessions.

Hope it helps.