i THINK thats its upto 3 times with retail, not too sure though.
I have an OEM version, and it only let me do the initial activation then I had to phone MS support every time I changed something after that

 

I talked to some more people too and they don't quite know the number either. To me i think it should be at least 20. OEM, that's another story, I understand why it's only 1 with OEM, but for the extra money you pay for retail it should be good size number. I guess the better reason for someone to consider retail is for the tech support.

 

i think changing a m/b you will need to phone each time