This will depend on the version of Linux you’re running; device files in /dev are often created automatically by the kernel and the number and naming scheme will vary. In RHEL and some IBM bullshit, this is entirely kernelspace (devfs?) and in 2.6.15+, this is mostly userspace (udev).
For example, *BSD and its cousin, Mac OS X, create device files in the /dev/diskxsx style: disk1s1 is the first partition of the first disk, etc.
Anyways, to answer the damn question, often there will be two letters added: /dev/sdaa1, /dev/sdab1. This happens on RHEL and SUSE, according to this authoritative-looking IBM handbook.
Don’t put complete faith in any of this—it seems like this is very possibly something that varies across architectures/drivers/distros/whatever.
lord I need to go to school now