You can have multiple accounts on one "trusted phone number". Trusted phone number is where Apple sends the SMS 2FA code. I have several Apple ID's on 1 phone number.
This is different than "Reachable at" phone number which must be unique and is used for iMessage and Facetime, and if it's blank other people can only reach you via iCloud account email.
(It makes sense if you think about it, parents setting up iCloud accounts for their children's iPads who might not have their own phone).
