Dear,

if you want to write servos, you can only manage two servos using the two PTO outputs
but if you want to Read only Servos you could manage up to 6 Servos, using HSC inputs

but of course it depends of the applications frequency

HSC or PTO up to 200Khz

you must see: http://support.automation.siemens.com/WW/view/en/41127453