You can install and activate Notes on up to 3 machines at one time.
Once you've activated Notes you can see how many machines you currently have activated if you go to Settings > Activation:
And, you can deactivate your computer in the plugin to free up your slots anytime.
Don't have that computer anymore? You can also clear a slot inside your FAW Account. Login, click the Activation Codes tab, and click the purple slot you want to free up: