What counts towards the 50 IP addresses?

I see several discussions about the 50 IP address limit but not an answer about what counts towards the 50.

From the wording in the warning about approaching the limit, I thought it was IPs being routed/NATted through the UTM.   I configured several devices to not access the internet, with no change in my IP usage.   I pointed them also to a separate DNS server that only answers local queries.  No change after that.

The next thing I was going to change was DHCP.  Do IPs getting DHCP leases count towards the 50?  If so, is there any service I've left out?

thanks

Kevin