Personally, I think you are exciting big time. Typically I only like 2-4 clients talking to a logix processor. The comms can strain the processor and cause lag, or unexpected results. BUT, that largely depends on how much data you are requesting, how efficient the code is written and your comms settings in your HMI.
You may want to consider an opc server, this would be a new computer that talks directly to the PLC, and the HMI's talk directly to the computer. Downside, if you need to reboot the OPC server, all your HMI's go down.