Although how much mulch fits in a truck bed depends on the truck’s dimensions, in general, three cubic yards is the upper limit.
A full-size pickup truck usually carries between two to three cubic yards of mulch. A smaller model could carry anywhere between one and two cubic yards.
Note that this formula will vary depending on the material in the load. Sand, gravel, soil, and mulch have different compositions and weights. You can carry less stone and topsoil than mulch, for instance.
Do you plan to pick the mulch from a garden or farm supply shop? Try asking the staff. It is probably not the first time they’ve helped someone get mulch into a truck!