Where can I find a list of chess games (or PGN file) for games less than X number of moves (specifically 12 moves)? I was reading a book on chess where the author suggested that a good way to practice getting good at the opening is look at miniature games (less than 12 moves specifically) played, that have led to checkmate.  The implication is that these games must have involved serious blunders to end so early, and so that an amateur can learn about vulnerabilities and how to take advantage of them, by reviewing or playing through these games.

I have looked up books on miniature games, but first they're often longer than 12 moves, and second, usually it's only the famous ones that involve brilliant tactics that are mentioned.  I want a much bigger list of these games that I can play through.  Appreciate any help.
pgn-extract can extract games from a pgn file based on the number of moves. The -bu12 flag would give you games of 12 moves and less. There are various databases which you can download for free, see this link

12 moves are really short games, you might want to extend that limit a bit and probably you also want to exclude games that end in draw.

Also IMO  solving general tactics puzzles is very useful for a beginner.