How to reduce RFID interference
I am using a MFRC522 RFID reader I have read that it's antenna when in
close proximity with metals then interference could be generated.

When the coil is close to a conducting surface (i.e. a metal part) eddy currents are induced in the metal and the coil's magnetic field overall becomes reduced. This is because the eddy currents act like mini-shorted-turns and the overall inductance of the coil reduces which can lead to significant detuning effects and loss of sensitivity. In addition, the eddy currents can also generate heat and that means that the magnetic field you are trying to generate becomes depleted (less effective).

What should be the distance between the antenna and the metal to
minimize the interference

Difficult to say because it's a 3D problem that is best solved using a proper 3D tool. However, you can buy flexible ferrite sheet that masks the effect of close by conducting metals and restores the magnetic field to the right level. In other words, for a given coil placed close to some metal parts you should experiment a bit. You can also use the ferrite sheet to enhance the field that your tag receives.