If I could only have one, it would be something like 1/2 - 5/8" wide. That said, I use a much larger one more frequently - 1.25 and 1.375" wide. I'd start with the smaller one.
Ultimately the answer is yes, multiple sizes are needed. That said, a lot comes down to what you are turning. I prefer the larger ones for roughing, and I generally don't use a Spindle Roughing Gouge, so the skew picks up that work. There are some places where I just can't get the larger skew into, so having the smaller one is handy.
Now, I'm ready for seeing 3 or 4 other answers in the next 2 posts!