Can you confirm whether you are connecting to a 3.5.0 server or a version 4 because this is not clear. From what you write it sounds like you are connecting to a 3.5.0, but it could also be a version 4 which has been configured to emulate a server 3.x.
Just to clarify what you are seeing in the Recent Connections list. These represent the connection files, and they are not a representation of how many sessions you have running on the server, or how many you are allowed to have. They simply represent the number of connections that you have run to a given server. (Step 2 here: https://www.nomachine.com/how-to-run-a-virtual-desktop-session). You have 4 different connection files for xx.x.xx.121 and only one with IP= xx.x.xx142.
You can click on one of them to connect to the server indicated in the connection information (Otherwise, inserting the IP in the white field evertime will create a new connection file). You will be prompted to log in. Once authenticated you will be presented with a choice of desktops. If you have a Gnome session already running, you will see it listed there. See Step 5 in the link just mentioned.
picked the session with IP=xx.x.xx.142 and was rejected with “OOPS :Reached the maximum of concurrent virtual sessions …..
That means exactly that. On server .142 you have reached the maximum allowed. To be able to provide further advice, we would need to know what product and version you have installed on both the servers you mention.