Several of the world's best known heritage sites admit only a fixed number of visitors each day, with tickets released months ahead. The limit is a conservation instrument, not a marketing device.
Bodies change the environment inside a building
People raise temperature and humidity in enclosed spaces, and the moisture from breathing accumulates faster than passive ventilation removes it.
In painted caves, tombs and frescoed chapels those swings drive salt movement and microbial growth in the surface layers, which is where the decorative material sits.
Because the damage is cumulative and slow, it is invisible over a single season and unmistakable across decades of monitoring records.
Wear on surfaces is mechanical and permanent
Stone steps, mosaic floors and timber walkways lose material to abrasion, and the loss is proportional to the number of feet crossing them.
Once a carved threshold or a mosaic tessera has worn away it cannot be restored without replacing original fabric, which conservation practice avoids wherever possible.
Caps, one-way routing and raised walkways all address the same problem by reducing contact rather than reducing interest.
Structures were not built for crowds
Historic floors and staircases were designed for the loads of the households or congregations that used them, not for continuous streams of visitors.
Concentrated loading and the vibration of many people moving together stress joints and fixings in ways the original builders never anticipated.
Structural engineers advising heritage bodies routinely set occupancy limits for these reasons alone, independent of any conservation argument about surfaces.
The number comes from monitoring, not guesswork
Sites with caps typically run instrumentation recording temperature, relative humidity, carbon dioxide and sometimes particulate levels throughout the day.
The cap is then set at the level where those readings return to baseline overnight, allowing the building to recover before the next day begins.
When readings drift, the response is usually to shorten visit duration or reduce group size rather than to close the site, since both reduce load without ending access.
Access is rationed rather than removed
Managers generally prefer timed entry, advance ticketing and guided routing over outright closure, because public access is part of why the site is protected at all.
Replica chambers, offsite viewing rooms and digital reconstructions exist to carry the majority of demand while the original carries a fraction of it.
The practical consequence for travellers is that the most fragile places require planning measured in months, and arriving hopefully at the gate rarely works.