Hi Rakesh,
It’s unlikely that we ever will have issues with MAC address exhaustion. 2^24 = 16777216, that’s close to 17 million unique MAC addresses for 1 OUI.
Within a broadcast domain, you need to have unique MAC addresses. Broadcast domains are typically very small so it’s very rare to see duplicate MAC addresses within a broadcast domain. If a vendor runs out of MAC addresses for their OUI, they probably can get another OUI.
Perhaps someday we run out of MAC addresses but I doubt we will still be using MAC addresses by then 
Rene