It is 24 with A trailer. 18 for the cab, minus 24 equal 6 so the tailer gives you 6 extra slots. If you have more then one tailer you have to keep adding the acel and manuver pelaties, of each tailer. So haveing three trailers gives you another 18 slots but triple the penalties, and keep in mind that you can not add engine mods to the trailers. One more thing tire mods don't take slots.