Can Apple add memory to my Mac Mini 2018? How much?

As we are fellow users, we can point you to sites where you can purchase the RAM, but if you do not plan to do it yourself, and complelely disassemble that mini to add the RAM, then you should contact an Apple Store, or Apple Authorized Service Provider to determine if it is a service that they provide, and the cost.

Maximum Memory 64 GB

Memory Slots 2 - 260-pin PC4-21300 (2666 MHz) DDR4 SO-DIMM (Upgrade through Apple Store or Apple Authorized Service Provider)

Or, you can DIY. Visit OWC for purchase of and help with increasing the RAM for your Mini.

DIY is certainly an option but you may wish to view this installation video to gauge you comfort level with the work.