There is no magic number in the third edition books. Based on the flavour of the game and the printed scenarios that I've seen for that edition, I'm happy with the following rule of thumb.
Make your players work for magic items by focusing entire adventures around them. This gives the players a feeling of achievement. At most, I'd sprinkle one to two per player throughout the entire campaign. In a 4 person group that might mean 1 or 2 every rank (a full career in WFRP 3E) of play.
I prefer WFRP without magic items though. The feeling of grim and gritty is short-circuited by having items of magic power. Those are for the Special Characters who lead armies. Not the guys who run around in mortal danger from the enemies of the empire.
Liber Carnagia: An adventure revolving around the Chaos spear Crimson Rain waking up.
The Game Master's Guide (Under Tomes and Artifacts in the Colleges of Magic chapter): The art of making magnificent and potent items like the Runefangs is described as lost to all but a few. All powerful magic items are extremely old and valuable. Today, minor magics like potions and charms are still created, but nobody really knows if they work or not.