Stars form from giant clouds of gas and dust, lots of material so can form lots of stars at once.
Some stars will wander off from the site of formation but sometimes stars will stay clustered. Open clusters are found in the plane of the galaxy. They contain up to several thousand stars, but usually in the 100s, size of 30 lightyears but are irregular in shape. Visible now, fuzzy-looking patch of stars. Globular clusters are found mostly in the halo of our galaxy, but also in the disk.