If it is an OEM copy - Activate it as often as you like so long as the motherboard is not replaced.
Once motherboard has been upgraded you need to buy a new license.
If it's a Retail copy - Activate it as often as you like. As with the OEM version it can only be on a single machine, however you can remove it from one machine, install it on another and reactivate.
Many of your future activations will probably require a phone call rather than an automated one online.