When you size your multiplexor, the calculation is based on your system load, particularly the number of concurrent connections the multiplexor needs to handle.
In addition, you must:
Add CPU or a hardware accelerator for SSL if appropriate.
Add memory to the machine if the multiplexor is being configured on it.
One or more of each type of machine should still handle peak load without a substantial impact to throughput or response time when you plan for redundancy in your deployment.