Alright, for starters, I'm not sure about the 250-route-limit you're talking about.
However, here's what I would suggest;
When you want to ask kernel-specific questions, include your Linux Distro's name, the version of your kernel, and what you're trying to do.
What you're trying to do sounds more like "tweaking" rather than "Oh, where's that routing.c file for the kernel?" Messing with one of the available settings in /proc sounds more up your alley.
Visit this site
, or read your kernel's documentation (as *that* would be the most relevant source of information).
Use iproute2 for any advanced routing.
If you're running Red Hat, CentOS, or something similar, there should be a package in your yum repository.
If you get completly lost, you can visit here
for an example of how iproute2 works.
Regardless, if you're new to Linux, and you're still not able to create 250 routes to virtual interfaces for testing, then there's one last site that might help
Be warned, you'll have to have a sense of humor.