Doors, fixtures, valves, panels and pieces of equipment are priced by the each. The quantity comes from a count on the plans or from a schedule.
Be clear about what one each includes. One door can mean the leaf only. It can also mean the leaf, the frame and the hardware.
Write the description so no one has to guess. For example: basketball backstop, ceiling suspended, with winch and safety strap, EA.
Check the unit on supplier quotes. A supplier may quote by the box, the pair or the set where you counted by the each.