As for my Tremere numbers, the 5% was based on numbers of the rough ruling class to commoner ratio at the time and I'm assuming that to be the point where it becomes blatantly obvious that vampires run the country. At that point or higher, there would be no real way to hide the Tremere in Transylvania. The actual number should be less than this although that would also assume that the Tremere didn't spread outside Transylvania.
Would you have any specific suggestions with regards to numbers?
The addition of it would make things more dangerous for the mages but wouldn't actually assure victory against Masters and Archmagi.
10 dice of countermagic to cancel a power or ritual that targets anything within a radius equal to his Willpower in yards or meters, or one that is being used in that distance would only be available to a handful of vampires. While it could be used to shutdown archmagi or master, this would assume the vampire can actually get within that distance of an archmagi before he or she creates a glowing orb that emits sunlight or summons a natural disaster. So while it makes it possible for the vampires to win against powerful mages, it in no way assures victory against the crazy tricks that an ancient an paranoid mage would usually have. On the other hand, 6 dice of countermagic against magic that targets a vampire or something touching the vampire would certainly be a serious problem for most Order of Hermes student mages and newbies.
It is my opinion that the initial attack against the Order of Hermes by the Tremere mostly killed the younger and less experienced mages of the order and much of their mortal compatriots (and maybe a few older ones here and there) but cost the Tremere dearly. It would have left a disorganized and greatly reduced Order of Hermes, however.