You can use Shift+Alt+B (Path > Trace Bitmap) in Inkscape with "Colors" checked, and adjust the number of scans. However, the gradients on the ocean and on the frame will be an issue as the process will take them into account before converting to a number of discrete colors. I think the best would be to start from the original globe image in Inkscape, vectorize it, and then add the frame and the gradients (you can have gradients in SVG but it's hard to convert pre-existing gradients to vector).

Brittany was one of the victims of the sea level rise on the icon. B(