Add slide about number of nodes in a cluster

slides/k8s/concepts-k8s.md

@@ -199,6 +199,30 @@ class: extra-details
 
 class: extra-details
 
+## How many nodes should a cluster have?
+
+- There is no particular constraint
+
+  (no need to have an odd number of nodes for quorum)
+
+- A cluster can have zero node
+
+  (but then it won't be able to start any pods)
+
+- For testing and development, having a single node is fine
+
+- For production, make sure that you have extra capacity
+
+  (so that your workload still fits if you lose a node or a group of nodes)
+
+- Kubernetes is tested with [up to 5000 nodes](https://kubernetes.io/docs/setup/best-practices/cluster-large/)
+
+  (however, running a cluster of that size requires a lot of tuning)
+
+---
+
+class: extra-details
+
 ## Do we need to run Docker at all?
 
 No!