Navigate to System → Advanced → Firewall & NAT.

Check the Firewall Maximum States and Firewall Maximum Table Entries fields.

Each State table entry requires about 1 KB of RAM.

• The default state table size is calculated based on 10% of the available RAM in the firewall.
• For example, a firewall with 1 GB of RAM will default to 100,000 states which when full would use about 100 MB of RAM.
• For large environments requiring state tables with several hundred thousand connections, or millions of connections, ensure adequate RAM is available.

Navigate to Diagnostics → System Activity.

Some packages increase RAM requirements significantly.

• Snort and ntop are two that should not be installed on a system with less than 1GB RAM.

https://pfsense-docs.readthedocs.io/en/latest/monitoring/system-activity.html