How thin can you pour concrete over existing concrete?

This answer changes depending on how you define things.

If this is a strength issue; you need to design the new slab over the existing.

If this is a finish or a wear surface it can get very thin. Some epoxies can get down to a couple of micros thickness, but that may not fit your criteria. Grout can get down to 1/8″ or less, but needs a suitable backing. Also, it is very fragile. Fiber reinforced toppers go from 1/8″ for non-wear to 3/4″-1″ for wear surfaces. Lightweight and regular concrete toppers tend to be about 1.5″ and do contain wire mesh reinforcement.

So, back to the question, how thin can I pour concrete over existing? Depends on your criteria, but if you don’t know, go with 1.5″ lightweight with a wire mesh. Don’t forget that if this isn’t a ground slab that you’ll need someone to do an analysis to see if you can support the extra weight. Any concrete adds weight to structure at alarming rates.