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Shirt #2 used to be worn by Bellerin. It's typically the number taken by a right back.
The current right back is Ben White, who wears #4. That's often a number worn by a CB, but sometimes by a midfielder. The other typical number for a CB is #5, but that's currently being worn by Partey, who is playing as a DM (Typically #6 or #4, depending on the country). Gabriel Magalhães wears #6, which is the other number that sometimes CBs use, if the #4 is in midfield.
The other number that defenders typically use is #3, currently Tierney has that one. But, if he's gone this summer, it goes up for grabs too. Does Zinchenko use it? Or will he keep his #35? The way he plays, it's reasonable if he doesn't wear the #3 shirt.
If Zincheko does get #3, the GK, back 4 plus Partey could all have the #1-6 shirts, but in a strange order.