How many colours should a trace have?

Set the count high and the tracer faithfully reproduces every slight variation in the source image. That sounds good. In practice:

Near-duplicate shapes. A region that reads as one blue becomes four blues, each a few percent apart, stacked as four separate paths with visible seams between them.

Jagged boundaries. Where two of those near-identical colours meet, the edge wanders pixel by pixel, because there was never a real boundary there.

An uneditable file. Hundreds or thousands of paths. Changing one colour means selecting dozens of shapes. The file is large and slow.

Anti-aliasing captured as colour. The soft transition at every edge becomes its own thin shape, outlining everything in a colour that isn't in the design.

Push the count down and detail collapses:

Lost distinctions. Two colours that mattered get merged into one. A logo with a navy and a black element becomes all black.

Blocky shapes. Shading flattens into hard-edged regions that read as a mistake rather than a style.

Lost thin elements. A fine light-coloured line surrounded by dark gets absorbed into the dark.

These are places to begin, not rules. Adjust based on what you see.

Logos and icons: 2 to 4. Most logos genuinely use two or three colours. If a trace of a logo needs more than four, either the source has gradients and shadows you should remove first, or you're capturing artefacts rather than design.

Flat illustrations: 6 to 12. Enough for a designed palette plus some shading, without fragmenting.

Detailed illustrations: 12 to 20. Start here and reduce until it breaks.

Photographs: don't. A photograph has thousands of distinct colours and no hard boundaries. Any count produces either a mosaic or a poster-effect approximation. If you want a vector from a photo, trace it as a deliberate posterised effect and accept that look, or trace it as a base and redraw.

Most tracers, Tracer included, don't give you a colour-count dial. You choose a preset and the preset decides how aggressively colours get merged. In Tracer that's Auto, Clean / Flat, Logo / Line art, Detailed, Sketch and Tattoo / Stencil, plus Threshold and Corner rounding.

So the ranges above are a target to judge the output against, not a number to type in. The loop is: pick the preset that matches the kind of artwork, look at the result at the size it will actually be used, and ask whether anything you need is gone. Not whether it matches the source, but whether anything you need is gone.

If a flatter preset loses something, move to a more detailed one. If a detailed preset gives you forty near-identical greens, go flatter. Stop at the flattest result that keeps what you need, because fewer colours means a smaller file, cleaner paths, easier editing, and fewer screens or vinyl sheets if it's going on a machine.

When no preset lands it, the fix is upstream, in the raster.

If you can't find a count that works, the problem is usually upstream.

Posterize before tracing. Reducing the image to a small palette in an image editor gives the tracer unambiguous regions to find, and almost always beats asking the tracer to figure it out.

Raise the contrast. Push the lightest tones to white, darkest to black. Edges become real edges.

Remove gradients. Flatten them to a single colour, or to deliberate bands if the design needs the transition. Either beats letting the tracer invent bands for you.

If the output is going on a cutting machine or a screen press, colour count is also a cost and labour decision. Every colour is a separate sheet to weed or a separate screen to burn. Four colours that look nice on screen is four pieces of vinyl to align by hand.

Tracer has presets that handle this for common cases, and SVG export is free so you can compare a couple of settings before committing.