Choosing the number of colours for a trace

The colour count is the number of flat fills the tracer reduces your image to before outlining each one as a path. Use the smallest number that keeps every distinct region you care about: usually the colours you can name plus one for the background. Going higher does not add sharpness — it adds thin edge paths from anti-aliasing; going lower merges neighbouring fills.

What the slider controls

Before any outline is drawn, the tracer reduces the image to n colours using median-cut quantisation without dithering. Every opaque pixel is assigned to the nearest of those n colours. Each colour's pixels then form a mask, and Potrace outlines that mask as one <path> with the colour as its fill. The paths are emitted largest-area first, so bigger regions sit underneath smaller ones.

So the slider is really "how many paths, and how coarse is the colour grouping". It is not a quality or detail slider.

The presets' starting points

Standard: 6
Cricut / Vinyl: 8
Line art / Stencil: fixed at 2 (black and white by luminance threshold)
Detailed artwork: 16

You can move the slider after choosing a preset; the range is 2 to 24.

A method that works

Name the colours. Look at the image and list the distinct fills you can see. Ignore anti-aliased edges and JPEG noise — those are not colours you want.
Add one for the background if it is opaque and you are not removing it.
Set that number and convert. Previews are free.
Read the palette strip in the result. If it shows a colour that is really an edge blend (a muddy in-between tone), go one lower. If two fills you listed came out as one, go one higher.
Read the path count. It should be close to your colour count on a flat design. A large gap means speckle paths.

What happens at the extremes

Too low. Neighbouring fills merge: a dark blue and a black become one black shape, and a light grey highlight vanishes into white. On a photo, 2–4 colours produces the poster look — sometimes exactly what you want for a stencil.

Too high. The quantiser spends colours on the anti-aliasing fringe between real fills. Each fringe colour becomes a thin path hugging an edge, so the file gets bigger, the shapes get a visible outline in a slightly wrong tone, and the editor's layer list fills with slivers. On JPEGs this also revives compression noise as speckle.

Special cases

Two-tone logos on transparent backgrounds: 2. Transparent pixels never count as a colour.
Illustrations with shading: the Detailed preset's 16 is a reasonable start, but expect visible bands where a gradient was. Use the SVG editor to recolour bands afterwards if the banding is acceptable but the tones are off.
Screenshots or UI mock-ups: often 8–12 works, because they are already flat; text will trace as shapes, not editable type.
Photographs: see the guide on why photos posterise; the colour count changes how many bands you get, not whether banding happens.

Holes and overlaps

Each path uses the evenodd fill rule, so the inside of a ring or a letter stays open rather than being filled — you do not need to raise the colour count to "see" a hole. Because larger regions are drawn first, a small fill inside a large one simply sits on top; that is why a two-colour badge on white can be traced with 3 colours and still look right.