In the book Pro SQL Server Always On Availability Groups They write about scalability. Here is what they say
Windows Server 2012 (and above), failover clustering supports up to 64
cluster nodes. This helps to easily scale out the solution by adding
nodes to it. For example, if you have a two-node failover cluster
hosting applications that is getting close to hitting a node-specific
capacity limit such as a CPU or memory, it is very easy to add a third
node and redistribute the load.
How is that possible? If you have one instance on two nodes and you need to add memory. How will another node help? You can't load balance to 3 nodes. I completely understand running two instances ( one on each node) but then adding a 3rd will provide what?
Best Answer
It doesn't mean that it provides every type of scalability, such as for adding memory and CPUs to a given instance. It means that you can scale up the overall environment by installing more instances that are spread across various nodes and have different possible owning nodes.