Two things get conflated: the download size and the free space the updater wants. The updater needs working room — it unpacks the update and, since iOS 9-era updates, can temporarily offload some apps to make room. Those apps come back automatically after the install, with their data intact (offloading keeps documents and data; only deleting removes them). One caveat: an app that has since been removed from the App Store cannot be reinstalled automatically.
There's no single safe number that works across releases, because requirements vary per update. Practical guidance: check Settings → General → iPad Storage — Apple's own article on needing space for an update recommends offloading apps rather than deleting them, and notes you can update via a computer (Finder on a Mac, Apple Devices app on a PC) if the iPad genuinely can't free enough space.
Running updates with storage nearly full works when the updater's offload path can do its job, but it's slower and riskier — if anything interrupts the process, you're more likely to end up stuck. Keeping a modest buffer (a few GB above the download size) remains the safer habit.
Sources: https://support.apple.com/108327
— Osnivo Support