Windows Azure provides us the ability to scale our application up by specifying how many CPU cores we want in our service instances, or to scale out by specifying how many single-core instances we require. Both strategies can be used to accomplish our scaling objectives for the same price (8 1-Core machines @ 12 cents/hour or 1 8-core machine @ 96 cents/hour), but in smaller deployment scenarios (under 8 CPU cores) there are a couple of advantages that clearly favor selecting a greater number of small-VM instances over a single VM instance with an equivalent number of cores.
The Windows Azure Service Level Agreement (SLA) guarantees 99.95% service uptime. To receive this benefit, the SLA requires that you deploy a minimum of two service instances. Another important feature is the Rolling Upgrade. A rolling upgrade is a deployment feature of Windows Azure that allows service instances to be stopped and upgraded individually without bringing all of your instances down at the same time. This allows your service to remain operational during upgrade periods (albeit in a degraded state).