There are 108 Upanishads in Hinduism. Of these, 10 are popular. Brihadaranyaka Upanishad is the largest. Isavasyopanishad is considered the smallest.
"Upanishad" is a Sanskrit word. Means: Sit together. With the teacher and learn. Upanishads are part of Aranyakas. What are Aranyakas?
Aranyakas means: Teachings in a forest. In those days, the teachers lived in forests.