Run a count query for every row ID in results
Im not sure if i understood correctly are you referring to a group by?    

SELECT lot.id,lot.name,manager.id,manager.name,count(car.id)
FROM lot
LEFT JOIN manager ON lot.manager_id = manager.id
LEFT JOIN car ON car.lot_id = lot.id
GROUP BY lot.id,lot.name,manager.id,manager.name;