Any number of nodes may be configured, although you need at least three total in order to tolerate a failure. You may also use any number of spare nodes. A spare node is a node that does not run any services until a failover occurs. If you have one or more spares, then they will be selected first to run resources in a failover situation. If you have no spares (or all spares are exhausted), then at least one node will have to run two services simultaneously, which may degrade performance.
The examples in this document will use 4 active nodes and 1 spare node, for a total of 5 nodes. Heartbeat has been tested with up to 16 nodes in configurations similar to the one outlined in this document.