Dns Socket pool enable your DNS Server randomizing the source port used to issue DNS queries to anothers DNS servers, is a great security feature against cache poisoning attacks. The number of a socket pool is 2,500 by default on the Windows Server 2012 R2.
You can get the number of socket pool issue this command:
>dnscmd /info /socketpoolsize
If you wanna reset this value issue this command:
>dnscmd /config /socketpoolsize 5000