Two Mesa 5I20

You can use as many PCI cards as you have PCI slots. They will have pins with names like hm2_5i20.0 and hm2_5i20.1

But, why are you using the 5i20? There are cheaper cards from Mesa with the same capabilities. (the 5i24, for example)

linuxcnc.org/docs/2.7/html/config/pnccon..._machine_information

PnCConf appears to understand how to configure 2 cards. If you need more than 2 then you would need to hand-edit.

Why do you need 2 cards? A single 5i20 (or 5i23, 5i24..) can handle 1152 IO pins using the smart-serial daughter cards.

You could swap one 7i37 for a 7i44 + any combination of up to 8 of these boards:
store.mesanet.com/index.php?route=product/category&path=83_88

If you want to use more 7i37 cards, it would be sensible to use a 5i24 rather than another 5i20, in any case.

Sorry, I am confused now.
Can you tell us what hardware you already have and how many servo channels and IO pins you need?

I see two cost-effective ways to add more IO.

1) Add a 5i24 and another 7i37TA = $198 for 8 outputs and 16 inputs
2) Swap an existing 7i37TA for a 7i44 and a 7i84. That is $148 for a net gain of 16 inputs and 8 outputs. (and another $79 gets you 16 more outputs and 32 inputs)

But: I think that the 7i37TA IO can be used for step-generation and encoder counting, whilst the 7i84 (and other smart-serial cards) can not. Though the 7i84 has two on-board MPG counters.