I always thought that the actual number was unspecified, so long as you comply with the spirit of the licence agreement which is that only one person can use it at a time.
Network rendering had a three computer use limit per single Vegas license. You'd need to buy another Vegas license to get another three network licenses.
I love this license they use: you can install it on any computer you use, you don't need to own them! :)
I've always found about 4 authorisations before needing to ask for more. Never had a problem with getting more, be it for computer rebuilds or new machines (I have multiple machines at home, a laptop, and one at my recording studio, etc).
However you are restricted by EULA to only one install being used at any one time per licence.
I don't see that it is restricted to one install, just one user per licence. For example I could run Vegas on my laptop and on my desktop at the same time, just as I could run two instances on one computer at a time. N'est-ce pas?
From the SCS KB ref no 4559 (see the link in a former post):
According the Sony Creative Software End User License Agreement, there is no restriction for installing or re-installing the software on multiple computers. Many users need to have the software installed on more than one computer (i.e., a home PC, a work PC, and a portable laptop) for the convenience of being able to work on projects at different locations. Our EULA allows for this type of use and only the purchase of a single license is required to do so.
If multiple, simultaneous use is needed, then according to the EULA purchase of multiple licenses will be required (i.e., a classroom or a business with multiple people using the same program at one time).