how do i add my wifes iphone8 to my apple account

It all depends on your goals. If you want to share things like App Store purchases, iCloud Storage Plan etc., you may want to consider Family Sharing: Set up Family Sharing - Apple Support

Otherwise, it is not recommended to share your Apple ID with another person. It will lead to issues down the road. Your wife should get her own Apple ID to keep things separate: How to create a new Apple ID - Apple Support