Thank you for choosing the best caching plugin, W3 Total Cache.
W3 Total Cache Pro licenses are valid on a per-install basis, meaning you need one for every WordPress instance you run. If you have three separate websites, you will need three licenses in total—one for each site. If you have five sites in a WordPress multisite instance, you need just one license for all five sites.
We do not currently offer any licenses for developers, however, if you are interested in bulk licensing, please email us for more information, and let me know the details so we can offer you the best deal.