how much ram memory can be added to a 2018 mac mini

This system ships with two 4 GB memory modules by default.

This model supports as much as 64 GB of RAM using two 32 GB modules.

Source of above info https://everymac.com/systems/apple/mac_mini/specs/mac-mini-core-i7-3.2-late-2018-specs.html

Additionally you'll have to have the upgrade done at an Apple store or Authorized Apple Service Provider. It's not a user upgradeable.