You need to run the
tag-pattern command. The default for that is
T (Shift+t). You can then give it a regular expression. By default this will match message subjects.
If you need to select a range of messages by number, you can provide the
~m [MIN]-[MAX] pattern to tag-pattern. There are many other options I've found useful over the years, and you can see a complete list in the “Advanced Usage - Patterns” section of the manual.
You can also use t to tag or untag the highlighted message, to fine-tune the selection.
Then you can run
tag-prefix (;) followed by
save-message (s), and it will prompt you for a mailbox name. This command marks the saved messages to be deleted; there is also the
copy-message command (
C, i.e. Shift+c) to copy without marking for deletion.