My baby 15" x 15" machine

[Jonathan]

It's hard to give a definitive answer as it depends on so many factors, what you need to cut and how often, how fast, required accuracy, personal preference... etc
It's borderline, you'd probably get away with one screw as the distance between the bearing blocks is fairly small, but if you're going to be cutting metals at all regularly or just generally want to be able to cut at a good material removal rate accurately then get two ball-screws. You certainly wont regret it.