You have probably seen the crazy numbers that a M2 Ultra Mac Pro can achieve. 24 channels of 4K60p! That's amazing, and we are so happy to finally have that computer available for our users.
In theory (but this was not yet tested), you could thus encode up to 48 channels of HD60p, all these channels running from the internal PCIe slots... and you could probably do more by using Thunderbolt ports.
But, there is one important limitation to know about: the ProRes hardware acceleration does not support interlaced formats, nor does it support alpha channels.
This means that if you are using formats such as 1080i59.94, or 1080i50, you will not be able to use the hardware acceleration for ProRes (you will be able to use the ones for H.264 or HEVC), and thus the encoding will be done on the CPU. In our tests, we have seen that a M2 Ultra Mac Pro was able to encode 22 channels of 1080i59.94 to ProRes HQ... still quite a nice number, but fewer channels than 4K60p...