As I read it, no you need a license. It is not the numbers, it is that you are using it for corporate use. The 500 is just an example; the key is that you are using it for non-evaluation purposes in a business.
See the full license. The summary is:
In summary, the VirtualBox PUEL allows
you to use VirtualBox free of charge
* for personal use or, alternatively,
* for product evaluation.
In addition, academic use of
VirtualBox is also permitted free of
charge by the PUEL.
If you do not belong to either
category, you will have to purchase a
commercial license. Do not hesitate to
contact Sun Microsystems, Inc. in this
This only covers the full package. If you only need the features in the VirtualBox Open Source Edition (OSE) edition that is under the GPL and does not require a license for corporate use.
If you are really looking for something free you do have a number of other options: VMWare Server, VMWare ESXi, Microsoft Virtual PC 2007, Windows Virtual Machine.