It seems that (given appropriate permissions I can send a PM to an unlimited number of groups because there is no code to check how many groups are being addressed.
For example, I set my maximum number of recipients to 2. Then I added three groups as recipients to a PM and was able to send it without problems.
A side effect of fixing this? Well, it seems to me that it would make http://tracker.phpbb.com/browse/PHPBB3-8510 reproducible.