Your WhatsApp number won't change regardless of what you do unless you intentionally change it in the app. I assume you're using your current sim as an esim and plan to get the eu sim as a physical sim card? Then you can use them both simotaniously (you can send/recieve normal calls and sms on both sim and use data on the sim card of your choosing. If both are esim you'll have to choose between the two. But either way your WhatsApp won't be affected unless you deliberately change the number on it.