How much storage does the iOS 14 update require?

You'll need roughly 2.7GB free on your iPhone or iPod Touch to upgrade to iOS 14, but ideally you'll want a little more breathing room than that. We'd recommend at least 6GB of storage to ensure you get the best possible experiences with your software upgrade.

https://www.express.co.uk/life-style/science-technology/1336239/iOS-14-Release-Date-Time-3-Things-iPhone-Owners-Must-Do-Before-Download-Update-UK#:~:text=You'll%20need%20roughly%202.7,experiences%20with%20your%20software%20upgrade.