Normally I agree with you directing people to Support for typical support issues or more complex general questions, but this is a typical question for a forum in my opinion. This type of question could easily be asked from someone looking into buying / choosing the product rather than an existing customer.
According to the help, the UA server supports <10 clients and every client connection increases resource utilization on the server side. For additional details and questions contact your Sales or Support representative.
The recommendations can be found in the Installation Guide, specifically for the OPCUA Server. The recommendations are intentionally a bit conservative. There isn't any hard limit on the total number of clients, it just comes back to resources (CPU, memory) on the local process. You need to monitor the impact on the Connectivity Server and IO Servers in your system as you scale up. One way to distribute such loads, is to create additional I/O Server and or Connectivity Service instances, thus providing more resource bandwidth. But it is best to do some internal benchmarking, refine your design, then implement. If in doubt, reach out to AVEVA support or your Technical Account Manager for further guidance.
