Hi! It is pretty easy, as long as you have a SIM card to begin the process. You insert a phone card and set Whatsapp normally.
After that you can remove the SIM card and even travel abroad, use another SIM card if you want and it will work normally (if the older number).
You will need a SIM card to begin the process, but if you remove the SIM card you can still use whatsapp on wi-fi no problem.
If you add another SIM card from your country or any other country later on you will still be able to use your old SIM card's number, whether you are using the phone internet or wi-fi.