We have 4 web servers and all of them serves one domain. But we are adding sub domains to IIS. Do you know what is the maximum sub domain count for IIS 7.5? Now we have 30 sub domains.
As Graeme pointed out, there's another good post on that here.
I'm unsure if your subdomains are setup as new sites or new site bindings, but either way IIS supports into the thousands of sites and bindings. A new site will have some memory overhead, but it's very minor. For .NET it's somewhere around 15-50mb of additional overhead for a new app pool plus the .NET framework.
Bulk hosters see the number of sites into the thousands so you are plenty safe with your configuration.