Freeing up space · Local snapshots and held space
How can I tell how much space my Mac's snapshots are holding?
Short answer: What's Using Space's Space held by backups section shows the total held and the oldest snapshot's age; the Local Snapshots window lists each snapshot and its backup status.
Two places show it. In What's Using Space, the Space held by backups section gives the headline: how much is held and how old the oldest snapshot is (or that they were all taken today). The Local Snapshots window lists every snapshot with its age and whether it's already on Time Machine.
The figure is the combined total macOS reports for all local snapshots; there's no per-snapshot size. See why there's no per-snapshot size.