What's the size of the environment you are trying to monitor in terms of:
- SAN/NAS disks
- Physical servers
- Virtual servers
The short answer is that a single STM server is limited to 500 server agents whether they are being used as proxy agents or not. However, from the way you've phrased the question, if you're trying to monitor just a virtual environment, I would highly recommend Virtualization Manager as it is much more scalable when collecting from a virtual environment and supports both VMware and Hyper-V.