According to polling, anywhere from 40-45%.
That said,what angry Berniebros say in March doesn’t necessarily mean it will be what they do after 8 months of media blitzing about how the world will end if they stay home.
I suspect that the Bernie Bros hate Trump more than your average rabid Democrat does.
I’m inclined to think it’s going to be a coin toss as to whether or not they will stay home