pfSense has the ability to collect DHCP lease statistics in it's Status -> Monitoring menu. That means it has some way to collect them.
searching through the source on github reveals dhcpd_gather_stats.php referenced in rrd.inc
pfSense runs this command to collect dhcpd lease counts from the lan interface: /usr/local/bin/php-cgi -q /usr/local/bin/dhcpd_gather_stats.php lan
and this for the opt1 interface: /usr/local/bin/php-cgi -q /usr/local/bin/dhcpd_gather_stats.php opt1