Revision: "per node" version ID
You can add an integer field to your node, and set the default value to 0.

Then in a small module, use hook_node_presave to increment its value every time the node is saved (or if you detect a field change). Quick, easy and efficient way.

You can do the same with a computed field, by incremeting the value of the field by 1 when you submit the form,  but storing code in the database isn't advised.