I know that we test QRadar with 1000 objects (network top levels) per our EPS/FPM testing certification document: https://ibm.biz/qradarepscertification.
I am not aware of a documented maximum, but I believe those 1000 objects that we certify against each have multiple CIDRs and are spread across multiple domains. I'm pretty sure I've heard of users that have upwards of 90,000 overall rows before, but never seen a NH that large myself.
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Jonathan Pechta
QRadar Support Content Lead
Support forums: ibm.biz/qradarforums
jonathan.pechta1@ibm.com------------------------------