How to rename many screenshots at once on a Mac

Rename a batch in Finder

Open the folder with your screenshots. Screenshots save to the Desktop unless you changed it.
Select the files. Click the first, then hold ⇧ and click the last, or hold ⌘ to pick them one by one.
Right-click any selected file and choose Rename.
Pick one of the three modes at the top of the window (below), then click Rename.

Replace Text

Swap one piece of text for another in every name. For example, find Screenshot and replace it with Lisbon trip.

Add Text

Add the same word before or after every name, such as a project or client name.

Format

Turn the whole batch into a numbered series, like Invoice 1, Invoice 2. You choose the base name and where the number starts.

Tip: renamed something by mistake? Press ⌘ Z straight away in Finder to undo the whole batch.

The catch

Finder gives every file in the batch the same kind of name. That works when the screenshots share one topic. When they are all different, a receipt, a chat, an error, you still have to name each one yourself.

Let each screenshot get its own name

Snaptitle looks at each image on your Mac and names it for what it shows. Its Tidy Up button does a whole folder at once, and good names are left alone. Every rename can be undone. See Snaptitle vs renaming by hand.