It depends. For a new application, you have no estimates about the number of users, number of pageview, response times and so on. I would start with one instan

It depends. For a new application, you have no estimates about the number of users, number of pageview, response times and so on. I would start with one instance. When the load gets high you can switch to a larger model. When you have collected some performance data, you can better decide how to scale. To give you some idea. I have a fb app running on my platform, that got 82000 visits last month. The avg response time is under 500ms. The instance runs on 2 cores.