Can the EasyPLC deal with a 64 input + 64 output Device?
What would be the top limit for the number of devices?
Theoretically yes, there would be no problem, but for the current drivers available only manages USB cards with 8I/8O (USB_OPTO_RLY) or 16I/16O Phidgets interface kit.
Take note that you can connect as much USB cards as you need to complete the desired I/O number.
Also remember that you can develop your own drivers for EasyPLC. If you are interested in manage a certain hardware you can do it, is very vell explained in the EasyPLC SDK guide.
Is not limit about how many devices can be managed, you can use as many as you want ;)