This depends on the number of connections associated with your Network Subscription. The NoMachine Network licensing model is based on the connections you run from one of your devices. If you have a Network Personal Subscription, one NoMachine Network connection is allowed, so you can reach one computer over the Network service. Just choose the device where you want to start the connection. 

If you have Network Standard Subscription, 4 connections are allowed. You can therefore, if you want, reach up to 4 different computers simultaneously over the Network service from one device. A Network Datacenter Subscription permits up to 32 Network connections from a given device and so on.

Remember that Network connections are connections which use the Machine Id to connect to the remote host. So your Network Subscription does not count the number of over-IP connections you are running from a device, ie. if you want, you can connect using the IP address of a remote computer from the same device where you start Network connections. 

More information about the different plans is available at the following page: https://www.nomachine.com/network/what-network-subscription.