iPhone 12 supports Dual SIM, although there are different ways of Dual SIM in different regions, some have physical Dual SIM and some eSIM plus a physical SIM.
An internal document posted by a user from the online reddit discussion area of a suspected telecommunications company mentioned whether 5G can support Dual SIM. It pointed out that when two 5G SIMs are activated at the same time, the two cards will also become 4G LTE at the same time. In other words, for the time being, iPhone 12 users can only use 5G with a single 5G-enabled eSIM or a single physical 5G SIM card. Inserting a second SIM card will change back to 4G LTE.
However, internal documents also stated that Apple plans to implement Dual SIM 5G through iOS updates this year.