This is dependant on the overall reason. for the slab, but the minimum should be 5cm. If the slab is thicker, this means it will be stronger and more durable, so for residential constructions, we normally go off the minimum of 10cm for a concrete shed or house slab.
A house concrete foundation might be thicker than a slab for a shed or a garage. It will require more cement and more labour, so will cost will be higher.