It depends what you're making. Some webapps might need 10 servers if you write it in php or ruby on rails, but may only need 1 server if you write it in c/java/something else.
For example, I could probably run Mibbit on around 50 servers if I just used apache/python/php/ror. And as far as I know that's pretty much what Meebo did. They had the money to throw at hardware. The alternative though is to re-architect and optimize at the early stages.
It's a longer term investment. I've spent a lot of time in my case, but I've saved a lot of scalability headache and cost, later on.