You can install this on as many of your computers as you'd like.
another question, will the future updates cost or by buying it once you have a life-time update?
At the time of writing this message MiniMeters is still in "public beta" so it's not even at 1.0 yet (0.8.5). You will receive updates from now through 1.x.x. I do not have plans for a v2.0 at the moment so likely you will continue to receive updates indefinitely.