it depends on the viscosity of the paint, the pattern of the door, aerosol or tinned and how the paint is applied, , ......but generally 500ml would cover between 4 and 6m2
It would only be enough if it was a small single door, otherwise you are going to need at least twice that amount. It is also best to put on 2 or more coats of paint.
If this is gloss paint on a well-prepared and primed aluminium door 500 ml is easily enough. It would help to mention the type of door (wood/metal?), its size, and the type of paint.
Most of the information you require should be wriiten on the can.