I believe that the 525 can only allow you to add two (for a total of eight) 10/100 Fast Ethernet ports. So you can only add 2 ethernet cards, of which if you want to use the full 8 ports would require two 4-port cards, of which only 6 ports will be active.
From Cisco: "The 525-UR adds the ability to statefully failover to a backup PIX firewall, includes an integrated VPN Accelerator Card (VAC), as well as support for an additional two (for a total of eight) 10/100 Fast Ethernet ports."
Hope it helps.
Steve