As many as you like. There is no site limit, no activation count, and nothing to buy again for the second site.
Why there is no limit
Everything we sell is licensed under the GPL. The GPL gives you the right to use the code, on as many machines as you want, and to pass it on. That is not a loophole. It is the license the developer chose when they built the plugin.
Per-site pricing is a business model built on top of a license that does not require it. Developers are entitled to sell that way, and plenty do. It is just not something the code enforces, and not something we are going to invent.
What that means in practice
- One plugin across every client site you build. No counting.
- Install the 99Plugs Update Manager on each of those sites, enter your key, and they all get updates.
- Development, staging and live copies of the same site are three installs and nobody is counting those either.
- Want to hand a plugin to a friend building their own site? Go ahead. We are not the GPL police.
The one thing that is limited
Nothing about the sites. What your purchase buys is the update service, and that runs on its own clock: a year for an annual pass or an individual 365 day purchase, forever for a lifetime one. When it ends, everything installed keeps working and only new updates and downloads stop.
Downloads themselves are unlimited too. See Are there restrictions on the number of downloads?
Do I need a license key for each site?
No. You can ignore the license key prompts entirely, whatever the plugin says and however many sites you are on. See My plugin or theme is not activating.