The answer is not simple. But, there is no hard limit maximum that I know of to the number of PLCs that FTView SE can talk to.
Yes, that will work with Siemens or Schneider PLCs.
There is a practical limit on the amount of data per second.
Talking to 100 PLCs at 1 tag per second is = 100 tags per second.
Talking to 1 PLC at 100 tags per second is = 100 tags per second.
It matters if those are scattered tags, versus array tags or in a UDT, or string tags. What PLC? Is it a ControlLogix L7? Ethernet through an EN2T?
Assuming you are only using scattered DINT tags, ControlLogix L7, a single EN2T, you could probably get 50,000 tags per second.
If those tags are optimized (array or UDT), then definitely that is no problem.