1)why does AHB or APB support 16 slaves devices?is there any calculation behind it?explain me?
I'm not sure that either protocol actually limits the number of slaves. Where did you see this described ?
I know that the original AHB protocol limited the number of AHB masters to 16, the reason being that AHB only supported one active master at a time, so if you had 15 or more other masters waiting to use that single bus you really should be looking at a different system architecture to improve performance and reduce latency for the other masters.
AHB-lite obviously only supports just one master on the bus, so no "16" maximum there.
For the number of slaves, there shouldn't be a limit. Connect however many you need for your application. Just remember that more slaves means a more complex AHB address decoder, so potentially starting to add a critical combinatorial timing path from HADDR to HSEL. But for APB the HSEL-PSEL timing is registered, so no reason to have any limit on the number of APB devices.
Please describe where you saw the slave number limit described so I can comment (if I can).