I always wondered why Sasuke has it both eyes, and Kakashi only has it in one, because they both have the same thing, right, so yeah
Sasuke is an Uchiha. He was born with both Sharingan eyes.
Kakashi is not an Uchiha, he was not born with any Sharingans. During a mission in his youth, one of his eyes was injured and he was gifted a Sharingan eye by an Uchiha teammate who was fatally injured (or so everyone thought - that became an important piece of a later story arc).