I would say that it is bad when it has large derivative (positive or negative). However, the problem is not about >number of human beings< but about making agency that existing people have obsolete.

We're making slave morality obsolete. The sense of self dependent entirely on external performance. Reality is making it so that we will have very little value in that domain very soon. Because everything we do now will be done better faster cheaper by the machines.

That's going to be a pretty rough transition. I think the economic aspects will be pretty straightforward by comparison to the psychological upheaval!

That depends on what you think jobs and the economy are for, generally.

If you think the purpose of the economy is for the economy to be good then it doesn't matter. If you think it exists to serve humanity then... You really wouldn't need to ask the question, I imagine.

The number of humans doesn't exactly serve humanity though either. Both of those variables are mostly irrelevant to actual human happiness and flourishing. In fact as far as I can tell they are actively harmful, for several different reasons.

It's bad if it goes down by more than about 1.2% per year. That would mean zero births, present-day natural deaths. Of course zero births isn't presently realistic, and we should expect the next 10-30 years to significantly increase human lifespan. If we assume continued births at the lowest rates seen anywhere on the planet, and humans just maxing out the present human lifespan limit, then anything more than about a 0.5% decrease means someone is getting murked.