I don't know what constrains the number of jobs--it obviously has something to do with the amount of capital, and the mostly accurate expectation the work done today will get paid well enough for more of it to be done tomorrow, but I don't have a feelng for the large picture. Maybe someday I'll reread _Man, Economy, and State_--I don't remember what Rothbard said about the question, but I'm pretty sure he addressed it.
However, I can think of a number of things worth doing which could work as jobs, which probably aren't going away anytime soon, and that people could get paid to do if we were a little richer or had a little more sense. It may be that people won't work as hard or as continuously, but there really are things for them to do.
Help desks could be better staffed with better trained people. Hospitals could be made into places that are good for sick people, though I grant that might lead to less work in the long run.
Buildings could be ornmented. I don't know why the Victorians could afford prettier buildings than we seem to be able to.
Clothing and shoes could be custom fitted--I'm imagining this done with considerable assistance from computers and automation, but needing human thought too.
There's science--we certainly haven't observed everything worth thinking about. Imho, one of the big frontiers is microbial ecology.
And even though everyone reading this lj probably has close to enough stuff, something like a billion people are living in dire poverty--supplying them with stuff will take a while.