Buying An iPhone 12, help.

So practically, me and my husband are going to buy my son a phone, and iPhone 12 to be exact, and so, we both are switching over to T-Mobile, now, since he doesn't have a phone number yet, do I have to come with a sim that has a new phone number to the Apple Store, or does Apple give you sims with T-Mobile that you can activate with a number? Like does Apple give an iPhone with a sim, that has a new phone number? Please let me know!

So do I come to the Apple Store with a Sim(that has a new number)

Or can I go to the Apple Store and buy a new iPhone that has a Sim(T-Mobile) with a new number?(Like do they have iPhones that have a new number?) This is hard to explain while typing, but I hope someone understands. Thank you!