Are you talking about 5-10k simultaneous users? It's very subjective depending on the usage profile of your site. The best thing is to pick a service like AWS/Heroku/Cloud Hosting Service that allows you to grow your offering. After you get to a point of profitability, you can look at greater efficiencies like setting up your own hosting hardware depending on your needs, if that makes sense. Our company has grown to a point where we are looking at implementing our own multi-datacenter cloud to save on hosting fees. But that's a good problem to have :D
