I'm currently in a game, where I have 5 mages on the board, but getting the 6 mage buff.
As you can see, LeBlanc is on my bench but the buff is still active. In the post match screen it also only counts 5 mages:
This isn't merely a visual bug as all my mages were in fact casting all their spells twice with one cast.
Not sure how to replicate... but it started when I had three mages on the board: LeBlanc, Vladimir and Taliyah.
Both Vlad and Taliyah were lvl 2, LB lvl 1. As soon as I got Veigar, I replaced LB with him. That's where the bug started, as I replaced LB with Veigar the Mage count went up from 3 to 4. Later on I got Brand and Syndra to get the full 6 Mage buff. This is also how I found out it's not just a visual bug.