Hmm, 1 GB of RAM should be able to run around +100 soldatservers (at least in theory), and I think that the CPU ain't going to really slow down anywhere around 10-20 servers.
It depends really what you need/want. If you clearly want to ensure that the hardware of the server AIN'T going to make ANY lag for the servers, i'd say that around 20-30 servers should be about the max. The second server should be abe to handle about 50-70.
Well, I haven't really played with that many soldatservers ever, nor with that kind of hardware, thus I'd recommend listening to FliseLikeABrick more with stuff regarding runnign multiple servers on specified hardware.