For a KDP paperback, spine width depends on your final page count and paper type. For example, a 246-page book on white paper with a black and white interior comes to about 0.554 in (246 x 0.002252 in).
Cream paper and color interiors use different numbers, so always check the current KDP cover calculator for your exact book before you finalize the design.
If the page count changes after the cover is done, the spine has to change too. That is why I always ask for the final page count first.