Prior to Chef Server 12, there were two version of the Chef server: open source, and enterprise. With Chef server 12, they were merged and the Chef server was made completely open source. If you are just starting to use Chef, you should use Chef server 12, as there is no reason to use an older version.
For any open source Chef components, you are free to use them on as many nodes as you want. This includes Chef server, client, and DK, as well as open source add ons such as push jobs.
The 25 node limit applies to closed source add ons, such as Chef Manage (the Chef server UI), Analytics, Reporting, etc.
Hopefully that clears things up for you.
- Mark Mzyk
Chef Engineering Manager