In ArcGIS server, default number of instances for each service is min=1 and max= 2 what will happen if the number of users is 10 in this case.
Each users request will wait for a free instance if one isn't available. It's important to know how much time it takes for the request to go through, (drastically different for GP than map services, for example), and then make sure you monitor the instances in use property within Server Manager and increase it if you see all instances being used. It may be good to look into load testing using JMeter or similar software.
Retrieving data ...