Why does my phone have two IMEI number and I have one SIM card?

Yes One UI 5 activated the second slot normally used for SD cards to be used also as a second sim Hence the two IMEI numbers

A mobile phone has one IMEI number per SIM slot, so if you have a dual SIM phone, it will have two IMEI numbers: IMEI 1 for the first SIM

imei slot 1 imei slot 2 2 IMEI numbers mean there is option that you can use 2 SIM cards phone numbers at the same time in your phone Depending on the SIMs slot if

imei slot 1 slot 2 1 2 3 4 5 6 7 0 Slot number 5 Base station transmit Corresponding receive frome 6 7 01 2 3 4 5 ' M ' in IMEI , number stands for mobile-