The release candidate of Windows 7 was available a few months ago, and that is valid until June/July 2010. What's the limit of how many machines you can install it on?
There was none as far as I am aware. The License Key issued could active multiple copies. I suspect there would be a limit at around a 100 if any.
I have used my key around 25 times during the RC on different machines and have not had an issue. Microsoft crippled the RC initially so the amount of activations is not a major issue for them.
If you just need a copy for testing purposes the 90 day free trial version of the RTM is a more appropriate choice.