I think the Airport Extreme can route more than 25 devices, however, I do not know the max. I did some Googling and 25 seems to what other's have observed with respect to Airport Utility -> Your AEBS -> Network -> DHCP Reservations.
I did find one post that said the Airport Extreme could handle 50 WiFi connections and 150 Ethernet connections, but there were not references sighted (I'm assume that all of these are dynamic DHCP assigned addresses or the devices using Manual fixed IP addresses).
May I suggest that if you have non-mobile devices that are not going to leave the LAN, that you give them fixed 10.0.1.* addresses outside of the DHCP range (just stay away from 0 and 255 (not sure if 254 is special or not). You can always change the DHCP range to give yourself more IP addresses that are not going to be assigned by DHCP. (Airport Utility -> your AEBS -> Network -> Network Options)
That is to say, a device that is always going to be at home and is not going to need to keep getting a new DHCP assigned IP address, can just use a fixed IP address via System Preferences -> Network -> Advanced -> TCP/IP -> Configure IPv4 -> Using DHCP with manual address (or similar for non-Mac devices).
Then just use the Airport Extreme DHCP Reservations for laptops and handhelds that leave the LAN and need to talk to get IP addresses via DHCP from other LANs.