A wagonmaker is a person who constructs and builds wagons.
The term you want is wheelwright."Wright," normally used as a compound, means someone who builds, makes or repairs things. So:"Wheel" + "wright" gives you wheelmaker."Wain" (wagon) + "wright" gives you wagonmaker."Ship" + "wright" gives you shipbuilder.
As many as he could afford. Really. As inhuman as it sounds, slaves were considered assets of the corporation that a Southern plantation was. Enough slaves were needed by the owner to produce the crops, keep the house running, and perform skilled trades needed (such as wagonmaker, wheel wright, miller, blacksmith, etc.). However, each slave resulted in Costs, such as food, clothing, housing, etc. Therefore, the slave owner, like any other businessman performed a cost-benefit analysis to decide how many slaves to keep at any given time.