I guess it really depends on the readers. There's a lot of fuss about GRR Martin, and if he does have a tendency to kill off characters unexpectandly, there are less murders in the books than in the series. And there are some author more prone to characters killing, as can be seen in many internet memes. Nevertheless, IMHO, the key isn't the death toll, but the explanation for it.
If you kill your characters by building some pressure, letting some previous signs, or with a plot logic and in-world logic, than a lot of people will go on. If you kill your characters for no reason, by lightning in a middle of a Sunny afternoon, when walking around: people might not understand. If it appears logic, then no problem. If it doesn't then it will bother people.
Again, back to GRRM. Note that he is well read. He does kill a few people, but there are mostly two death that are of importance, namely
"Ned" and Robb Stark.
and those are made for a plot reason
To destabilise the readers by removing any (main) hero. The honest-to-God good willing hero,
or as he puts himself
"I killed Ned because everybody thinks he’s the hero and that, sure, he’s going to get into trouble, but then he’ll somehow get out of it," said Martin.
"The next predictable thing is to think his eldest son is going to rise up and avenge his father. And everybody is going to expect that. So immediately [killing Robb] became the next thing I had to do."
But on the top of that, it is realistic in the given world, and has some logic behind it, like
folly and vengeance.
For me, gratuitous things are more a problem. I had problems to start the series, as it left most of non-violent, non-sexual content out.
At what point does it bother readers? Well if you do kill one important character in the middle, you will loose some readers. This is inevitable. The more the reasons for it are clear, the less you loose.
But you might consider the "trauma" of Aeris' death (in Final Fantasy) to see that one important character is often enough to mark the audience.
So it really depends on who you are writing for.