This depends on several factors, such as the hardware specs of the scanners, the number of network scans run, the type of policy used, the systems on the network, and for the PVS side, the amount of traffic generated.
In practice, I've seen generally one Nessus scanner (6.10) with the suggested required hardware specs doing daily credentialed vulnerability assessments on a local network can handle about 1000-1500 hosts. This assumes reasonable times for scans (couple of hours). That said, this greatly depends on a variety of factors, including how quickly you want your scans to complete.
One (1GB - Standard) PVS with the minimum config can reasonably keep up with about 600-800 MB/s of network traffic off a tap. More than that and it will stop to drop packets. Different types of hosts produce different types of traffic, so this is harder to measure from a # of hosts perspective. If you have insight to the bandwidth on the segments you want to put PVS on, this will help you design the deployment.
Also, review the minimum hardware requirements documentation as well as the Security Center and PVS architecture documentation, this contains helpful information.