Requested feature overview description.

An estimated amount of changes/affected posts when pressing Preview on the mass-edit page.

Why would it be useful?

The mass-edit tool is a dangerous tool, so naturally you're already checking your input half a dozen of times. Even then, being able to quickly see if you did it right (0 or 146263 changes is likely an invalid input) may prevent any unfortunate future mishaps.

It also helps forming an estimate of the size of the job; big jobs could impact the site's performance, and if an estimated numeric preview indicates so, range syntaxes can be added to split the jobs up in pieces.

What part(s) of the site page(s) are affected?

Mass-edit tool.