Do I need a license of After Effects for every render machine

No.

As of 2020, Adobe’s licensing allows you to have unlimited render machines as Render-Only nodes. This means you can use After Effects to render without being signed into Adobe Creative Cloud.

You only need to log into Adobe Creative Cloud to install After Effects. If you’re using third-party plugins, make sure they’re installed and working, then you can sign out of Adobe.

Then inside the Render Boss app open the Options  and go to the Render tab. Then select the option Render as Render-Only node. 

With this option checked, you should be able to use Render Boss on this machine without the Adobe sign-In pop window showing up. And since you’ve signed out of Adobe, you can use that account in a different machine.

Note: If you open After Effects while Render Boss is rendering, it will open in this render-only mode where you can only use the Render Queue and nothing else.