You should be able to input the full number despite the box not being big enough although last I checked, xbox ports for live were:
* TCP 80
* UDP 88
* UDP 3074
* TCP 3074
* UDP 53
* TCP 53
You have to do each port as 1 entry.
Your other option if you're not forwarding ports to anything else is to simply DMZ your xbox which is what I did. Works great.