Best practicies show that one probe shouldn't be used for discovery of more than 5000 IPs. But this is very rough number. It's all depends on the pattern of usage and amount of CIs discovered for every host. For instance, assuming amount of CIs coming from inventory discovery 1500 IPs per probe sounds as bullet proof number.
Hope this helps.
Regards -Dmitry Gomel, PMP If you find that this or any post resolves your issue, please be sure to mark it as an accepted solution. Click the Like button at the bottom to say 'Thanks'.