SQL Server 2014 instance count
You've asked a subjective question, but you'll find that many of us will say that the best practice is to have only one instance per node if it's a production cluster. If you have two instances, then many of us will say you need 3 nodes: one node for instance1, one node for instance2, one node is passive and is ready to be used if either of the other two nodes go down.

Brent Ozar recently blogged about this instance stacking topic. There are benefits and drawbacks to instance stacking. One such benefit is licensing and one such drawback is performance tuning. 

Disclaimer: I work for Brent Ozar Unlimited. The referenced article is from our web site and is written by my boss.

SQL Server has supported mount points since SQL Server 2005, so drive letters didn't limit the number of instances even ~12 years ago unless you got really crazy with tons of instances in a cluster. 

I have never heard that there's a 5 instance per node best practice for 2008 R2. I have had 4 instances on one node in a test environment, but the databases were tiny and very little utilization. I did not have to worry about performance tuning on that server at all. 

Now if you were to change your question to ask what is the maximum number of instances you should have on a node, then you might get a different answer. But that answer is going to be dependent on the hardware of the server and workload of the instances. I still say 1 instance for each node though if it's a production cluster.