I tried installing the beta on my main PC after installing it on my laptop (both of which have Unity 4, only one used at a time, obviously) and was told that I had reached the maximum number of installations. Is this a bug or something peculiar to the beta, as the T&Cs in the installer have the same clause as Unity 4 had?
fwiw, I have Unity 5 beta installed on two machines. You can go to your account page, and check the serial number for activations here:
https://store.unity3d.com/account/licenses
Then disable all current activations, and then just reactivate the two you want.
It seems to count 4.x activations as different from 5.0b activations. For now, I just reset them, but hopefully this will not be the case on full release (I need both versions active at the same time, as I’d expect most of us do).