A lot of devices have this limitation, the #'s very on how many MAC's can be entered but they all have this issue. The good news (or bad depending on how you look at it) is that MAC filtering doesn't really provide the security that most people think it does (easily sniffed and spoofed). I would suggest switching to encryption, while WEP and WPA has been proven compromised, WPA2 and more specifically AES are still very strong encryption's and should provide the security you require.
The basic reason is that MAC only filtering shouldn't be done on a large scale, and thus why you see such limitations.