Thanks for answer, What I need in fact, Is where to configure gdm ou maybe other package (xdm or ...) , to increase the number of connexion to the server.
Now I can just connect 15 light client ( I am not the person who configure firstly the server, I just start to do), I suspect that the previous administrator has limited the number of client connexion , BUT WHERE !??. the clients ( neoware box) use XDMCP protocole. thanks for help best regards