That seems like a remarkably non-scientific way to measure how many Battle Mages died in that 10 year span. Roswita did not sit next to each and every one of them including all the ones that undoubtedly blew up or worse, because that is how battle magic works. 20% could have died and been replaced in that time and as long as that is not an unusual replacement rating Mathilde would not have remarked on it for the same reason she does not constantly remark that the sky is blue.
So my pulling actual numbers that have been shown in the quest, which come from a specific inquiry into whether there have been any problems with the Battlewizards, is deemed "unscientific"
Whereas you are supposing 20% of them blew themselves up based on no evidence in particular?
Not to mention supposing that every single death must have been via miscast as opposed to angry Vampire?
There are from Mathilde's own estimates, which Boney has stated OOC to be his own back of envelope maths, 400 Magisters or above in the Colleges.
That is roughly 50 members of Magister level or higher per College
That's not a whole lot of Wizards
How many of those do we suppose are Battle Wizards? Just 10 would be 20% of each College
Because if the numbers are low and there's only a couple of Battle Wizards then they're a rarity and the numbers don't support them being high turnover, extremely replaceable assets. They can't be dropping like flies if it's going to take 5 years of training to replace one and you've got only 2 or 3 of them
Plus however many years it takes just to find someone with the potential tobecome a Battle Wizard, only one in thirty thousand go on to become Magister level
If Battle Magic is as rare as you make it out to be then an even smaller fraction of that is your recruitment pool
That'd leave significant periods where there's no Battle Wizards in the College period, which is then failing it's obligation to produce relevant war assets
And if it's the other way around and a very significant chunk of Colleges are Battle Wizards then that suggests that the occupation isn't very lethal for entirely different reasons