There are a few different factors that go into the answer to this question. The most important factor is the size of the hauler.
Larger haulers can typically accommodate more slots than smaller haulers. However, there are also other factors that can affect this number, such as the type of cargo that the hauler is carrying and the hauler’s configuration.
In general, most haulers can accommodate between two and four slots. Some larger haulers may be able to accommodate up to six slots, but this is rare.