A classic French croissant has 55 layers, according to the traditional layer count associated with Larousse Gastronomique.
Those layers are created by laminating yeast-leavened dough with butter. The dough is rolled and folded repeatedly so that dough and butter form a thin, alternating structure. In the traditional calculation, three folds produce 55 dough layers, with butter separating many of them.
The number is a useful culinary reference, not an absolute physical constant. Bakers may use different folding methods, and modern recipes sometimes produce 27, 81, 109, or other layer counts. Layer counting also varies depending on whether touching dough sheets are counted separately.
More layers do not automatically mean a better croissant. Excessive folding can make the layers too thin, damage the butter structure, and reduce the open honeycomb crumb that defines a well-made croissant. Temperature, butter consistency, proofing, and baking are equally important.