Hi @Gaurnag, welcome to the forum 
This post is the main thread on that topic, and may be worth a read (including some of the other posts it links to).
If you’re planning to use a single relay for switching the battery power then ideally the relay will be rated to at least the output capacity of the battery you want to use. A minimally sufficient option would be rated to at least the amount of power you expect your vehicle to use (which is dependent on what hardware you have installed, and how you’re planning to operate it).