Aximmetry Professional Edition supports a total of 4 SDI input/output ports per machine that includes both the input and the output ports. It means that if you need at least one SDI output you can only receive a maximum of 3 SDI camera inputs.

Aximmetry Broadcast Edition does not have these limitations.

Alternatively you can build a multi-machine system with a Professional license per machine, and for e.g. use a dedicated machine for each of your SDI cameras.