PC deaths should be based on bad luck and bad decisions, not character level.
So I picked "multiple per character level," guessing that's how often some PCs can make bad decisions.
I generally agree with this idea. But I will posit that decisions can be risky/deadly without being bad, per se. Just as the DM has some control over how difficult encounters are, players have some control over how much risk they want to take. It's perfectly fine for one group to rush into battle where another would prep first or negotiate.
All that being said, I like my games at least a little risky. I'm okay if death never happens in a game, but the opportunity for death should present itself often.