I think you should restate your requirements.
The number of clients is not the correct end number you are looking for, it is the number of states (RAM usage). I have about 20 devices on my network doing things. I have 800 states, or about 40 states per device. 10k clients times 40 states is 400,000 states. My 8Gb FW defaulted to over 800k states, my memory usage is low so there is room to grow. (40 states per device is not a solid number you should use, just an example.)
10Gb is where you look at interfaces and CPU, a 6100 should handle that speed.
Any packages you add on top should be added to the CPU and RAM numbers.
When you are done you should have an idea as to the the CPU and RAM needed, then you can make an educated guess on which device is best suited for your needs. If you have performance data from previous events you can improve your estimation on CPU and RAM needed.