you need a router...whenever you have to route data from subnet to subnet. The other thing a router does is divide the network into multiple broadcast domains.
fyi, I have run more then 600+ boxes on a single subnet (not that big a deal). you are thinking 255 because of class C addressing. 10.0.0.0 subnet (class A) with Subnet mask of 255.0.0.0 has a few more then 255 (10.0.0.1-10.255.255.254, and every one in between)