Working memory holds four chunks. Throw eight unrelated items at a person and one drops.
A six-digit number gets remembered by chunking — three pairs, two triples, whatever lets the brain hold the pieces while it processes the rest. The chunking is involuntary. The four-slot ceiling is the bottleneck the schema is built around. Anything past four either gets dropped on the floor or rolled up into a chunk and counted as one. Designing a page is choosing which four chunks survive.