I don't think there is an activation limit. I have never seen any documentation from Microsoft indicating this is the case. The problem is your activation was tied to your initial computer and certain HW upgrades makes Windows think you're using Windows on a new computer.
If you do find yourself with a non-genuine version of Windows after performing a hardware upgrade, replacing a failing component, or — in an extreme worst case scenario — upgrading a hardware driver or firmware that causes Windows to think you’re using a new component, you’ll need to activate Windows again with Microsoft.
To re-activate a Windows 7 PC, you’ll need to open the Windows Activation tool. To do so, click the Start button, type “Activate” into the Start menu, and click the Windows Activation shortcut. Select the “Show me other ways to activate” option and go through the wizard, selecting the “Use the automated phone system” option. You’ll then be given a toll-free number you can dial. If everything goes properly, you’ll be given a confirmation ID you can enter into the Windows Activation tool to make Windows genuine again. If the automated system fails, you’ll be put on the line with a Microsoft customer service representative. Explain your situation and they’ll hopefully allow you to re-active your installed copy of Windows, making it genuine again.
Taken from here