Somewhere, someone has taken the ugly Christmas jumper to its absolute logical extreme, and it has been officially measured while they did it.
3,473 people, one basketball game
The record for the largest gathering of people wearing Christmas jumpers is held by Kansas Athletics, who assembled 3,473 people in matching festive knitwear at a men’s basketball game against Montana, in Lawrence, Kansas, on 19 December 2015. Getting that many people into jumpers at the same event, in the same place, at the same time, and having it verified, is a genuinely large logistical undertaking — and a reminder of how far the tradition had already spread by the mid-2010s.
Source: Guinness World Records
Fifty jumpers, one person, one sitting
A separate, considerably more eccentric record covers the most jumpers worn at once by a single person: 50, achieved by Skyler Rozell-Whitaker in Monroeville, Pennsylvania. Layering fifty jumpers on top of each other is not, strictly speaking, a Christmas record — the category covers jumpers generally — but it tends to get attempted with festive knitwear precisely because that is where the largest cheap supply exists.
Source: Guinness World Records
Why records like this exist at all
Mass-participation and endurance records are attractive to the same instinct that drives an ordinary office jumper day — a low-stakes, low-skill way for a large group to do something visibly ridiculous together, usually for a good cause or simply for the story afterwards. A basketball arena of matching jumpers and a single desk covered in tinsel are the same impulse at wildly different scale.
You do not need a record to make it memorable
Most offices are not attempting 3,473 people, and most competitions are decided by a single judge glancing round a room rather than an official adjudicator with a clipboard. The same principles apply at any scale — visibility, commitment, and a design people actually remember. How to win a Christmas jumper competition covers what that looks like at office size.