There are multiple Checkmk editions. Currently there is RAW, Free, Enterprise and Managed Service. You can read more about the editions and versions here:
The Checkmk Free Edition is the Enterprise Edition with some Limitations. You can use the trial period to fully test the Enterprise Features without limits. After the trial period it will be limited to 25 Hosts.
Here is it from the FAQ:
Frequently asked questions
What happens once the trial period expires?
Your trial version will revert to limited functionality. You’ll be able to monitor a maximum of 25 hosts, and distributed monitoring won’t be available.
Here you can find the FAQ: Checkmk Trial - The IT Monitoring Platform
Hope this helps.
Regards
Norm