Article 24(1) of the Visa Code, as amended in 2020: "the period of validity of a visa for one entry shall include a 'period of grace' of 15 calendar days", which states may withhold for public-policy or international-relations reasons. A single-entry visa for a ten-day trip is therefore normally valid for 25 days, but the duration of stay printed on the sticker still governs how many days you may spend inside the Schengen area. See single vs multiple entry.
The three numbers on a Schengen visa sticker are independent: the validity window, the number of entries and the days of stay. Schematic of the uniform format; sample data fictional.