15 (fifteen) is the natural number following 14 and preceding 16.
Seashells from the mollusk Donax variabilis have 15 coloring pattern phenotypes.
11 (eleven) is the natural number following 10 and preceding 12. It is the first repdigit. In English, it is the smallest positive integer whose name has three syllables.
Copper engraving of a hendecagon, by Anton Ernst Burkhard von Birckenstein (1698)