I recall i read somewhere about 11n or 11ac standard that as per standard, total number of associated clients per radio should be 255. Is this true? coz i am not able to find any such reference anymore if i google it.
Any ideas or pointers will be really helpful
The theoretical maximum has always been about 1009 clients or something like that (its been way too long since I knew that number). Of course it's never been that high in practice.
There are several kinds of limitations, I would suppose you are referring to performance limitations due to memory size or CPU horsepower in the AP. Those have always been design/cost limitations, which may or may not have been admitted to by the manufacturer.
Other limits are imposed by the total bandwidth supplied over the Ethernet WAN cable(s), AND the total load imposed by the clients.
Other limits have to do with algorithms covering such things as Power Save, connection Ageing, and other considerations.
GUI controls in Enterprise and SOHO AP's often allow the Administrator/owner to set upper limits.
What happens when these limits are reached CAN be an important consideration. For example, does the AP simply stop admitting new clients, or does it actually refuse connections with a status code that says "no further resources available". Most newer enterprise AP's use the latter approach, but not necessarily all of them.
Sorry I couldn't give you a better answer than "it all depends...", but that's probably the best possible answer. If you are looking into a specific model and manufacturer, I suggest you get a hold of one of their technical desk(s).