That sounds like a limitation of Webmin, not of Linux's routing capability. Try running
ip route add 1.0.0.0/8 dev eth0
and see if it gives you any problems. That network isn't in use on the greater internet, so it shouldn't introduce any major routing issues if it works.
As an aside, if you're trying to maintain that many static routes, consider setting up a dynamic routing daemon like
Quagga. It should easily scale to 20 or more routes using OSPF or BGP. If you have any experience with Cisco hardware, you should have no problems.