Sunspots typically about 10000 km across. At any time, the sun may have hundreds or none. They are dark color because they are cooler than photospheric gas (4500K in darkest parts). They look dark only in comparison with the brighter and hotter regions of the photosphere around them. Each spot can last from a few days to a few months. Galileo observed these spots and realized the sun is rotating differentially (faster at the poles, slower at the equator).