Apple will optimize it so they don’t enter markets that would cause their tax bill to rise. So say Spain wants 25% and other countries just want 20%, well, iPhones will just be sold in the black market like they did in India, so Spain loses out on the VAT that it is already reliably collecting. Each country also wants Apple to compute their profits differently, and they will probably argue about what profit they have dibs on (if multi taxation is avoided).