I recently needed to tag the first 5,389 messages in a mailbox using Mutt. This is much easier than you might think.

1. shift-t - By default, the command to tag messages based on a pattern is bound to shift-t. If you've changed this in your muttrc, use whatever you've bound tag-pattern to.
2. ~m1-5389 - ~m is the pattern indicator for message ID. It can accept a single value, or a range of values. In my case, I've told it to match messages 1 through 5,389.
3. Don't forget to use the tag-prefix key (; by default) to perform an action on the messages you've tagged.