On cloud services web roles you must create at least 2 instances to get Azure's 99.95% SLA because when they do updates to the servers' OS etc, Azure will need to restart the machines (one at a time). This is well documented and you will be shouted at by the portal when running a one instance web role for doing this.
With Azure websites you don't get such warnings and I cannot find any documentation suggesting 2 instances for the 99.9% SLA. However, I also can't find any documentation saying that you don't need 2 instances for this SLA.
So which is it? Do I need 2 instances for Azure websites' SLA like web roles or is that a thing of the past?
A developer on the Azure Websites team has replied here that
the SLA for any Azure Website on the Basic or Standard tier is 99.95%
Just having one instance of the site is enough
You can download the Azure Web Sites SLA document here: http://azure.microsoft.com/en-us/support/legal/sla/
It does not state that you need 2 instances to get the 99.9% SLA but clearly state that the Free and Shared Web Site Tiers are not covered by this SLA.
We guarantee that Web Apps, Mobile Apps, Logic Apps, and API Apps running in a customer subscription will be available 99.9% of the time, with a 99.95% SLA provided when multiple instances and traffic manager are used. No SLA is provided for Apps or sites under either the Free or Shared tiers.
来源：https://stackoverflow.com/questions/26587234/do-you-need-2-instances-of-azure-websites-for-sla-like-web-roles