The Nagios XI free license key allows you to monitor up to 50 nodes for personal and non-commercial use.  There are no node limitations in Nagios Core.

If you would like to purchase a commercial license for more than 50 nodes, you can do so by visiting www.nagios.com/products/nagios-xi/ or contacting sales@nagios.com

You can find this information along with other info at our faq page here- https://www.nagios.org/faq/

Let us know if there is anything else we can help with.