pfSense

Versions 2.* have a drop-down option under DNS Resolver -> Advanced:
"Number of Hosts to cache"

The maximum number of hosts presented is 50,000. This value is updated in /var/unbound/unbound.conf:
infra-cache-numhosts: 50000

Several of our servers appear to be exceeding this:
msg.cache.count=96942

The server referenced above is an R320 with a 10gig NIC. It frequently passes over 1Gb of traffic with several thousand clients.

Additionally, several tuning guides recommend increasing this value to 100,000. Here is one of them:
https://calomel.org/unbound_dns.html

Rather than hacking something outside of the webGUI I feel it'd be of value to add 100,000 as an option in the drop-down.