There seems to be a misguided theory in the industry that the maximum number of devices in a zone is 20 according to SANS10139. THIS IS INCORRECT! There is no part of SANS10139 that states this. In fact the maximum number of devices per zone is 32 and is part of the EN54 standard.
We understand where someone has used the maximum coverage of a smoke detector is 100 square metres and that the maximum size a zone can be is 2000 square metres and taking this with no design consideration equals a maximum of 20 detectors. However, very few detectors get to achieve their maximum coverage due to partitions and walls in buildings. In addition not all devices are smoke detectors some are Manual Call Points or other detector technologies and therefore a zone can quite often be above 20 devices per zone!