Each tuner allows a single channel to be watched by a single workstation.
So if you have several computers, you will very unlikely be able to get by with one or even just two tuners. As more and more people go for multi-tuner setups to watch and record channels at the same time, I can't believe they just recently introduced a single tuner version. I see no reason why this would be a consideration, even for a single computer--because then there is no record & watch another.
We have two boxes with two tuners each. It isn't tough to see two tuners in use and that would be bad if that was being at capacity. I could definitely see a future need for adding a third box. But for right now, we're fine.