Your short-term memory holds about seven items, give or take two
Psychologist George Miller noticed that people's immediate memory for lists of digits, letters or words consistently clustered around seven items, almost regardless of how much information each item carried. The trick to holding more is 'chunking': grouping items into larger meaningful units, so a phone number becomes three or four chunks instead of ten separate digits. Miller himself later said he treated the number seven as a catchy rhetorical device, not a strict law of the mind.