Does anyone know how to get the serial number of the mavic mini? Is the serial number the flight controller serial number? Because in the app there is no serial number for just the drone.

Read the S/N off the box that the Mini came in, or read it in the micro-print label on the inside of the battery compartment door (or read the QR code).  That is also listed in the app under Device Management as the Aircraft SN.
Just to confuse the issue, there is another serial number listed in the app under Device Management for Flight Controller SN.  That is also the SN listed for Aircraft for your flights in the Flight Data Center.  That is also listed in the app on the About tab for Flight Controller SN.  Some people have referred to this as the "Electronic S/N".
That same location on the About tab also lists the Battery, Remote Controller, and Camera serial numbers.

Hi, AidanDroneFan. Thanks for reaching out! Flight Controller Serial Number is different from the Aircraft Serial Number. Aside from what you can see in the DJI Fly, for the Mavic Mini, the aircraft serial number can be found in three places: on the propeller sticker of the new aircraft’s propeller, on the sticker inside the battery compartment rear cover, and on the bar code sticker on the side of the package.