Fewer people are needed to perform a specific job as we do it today. But there will always be room to improve the products and services we produce and to make them cheaper.
Consider the steam-locomotive. It was a great invention but we didn't stop there now we have bullet-trains and hyperloops.
There is also plenty of room at the bottom as famously noted by Feynman I think. We can make products smaller to make them easier to carry around. New cheaper price points create new demand and new markets.
Also as economies improve, people will have fewer children, because they don't need them to take care of themselves at old age. So it's not like we will have too many people having nothing to do.
On the contrary I think AI can be the great equalizer: Rather than having fewer people who know how to do things, we will have more people who are able to do things with the help of AI.
The technology by itself will not produce a great society however. We need democracy to accomplish that. As people will have more free time they will have time to study and more and more people will demand democracy because they will understand it's the only way to avoid wars.