Taxine alkaloids[0]
The estimated lethal dose (LDmin) of taxine alkaloids is approximately 3.0 mg/kg body weight for humans.[27][28] Different studies show different toxicities; a major reason is the difficulty of measuring taxine alkaloids.[29]
It goes on to say that rats are ~20mg/kg, which would put a human at somewhere less than 1.4grams.Which is close enough to, "any exposure at all will kill".
> Which is close enough to, "any exposure at all will kill".
How much is in one seed?
I could only find a few sources saying that you would need to eat about 50g of the needles to reach the LD, and that's... A lot. There's no way a child would accidentally manage that, for example (even assuming LD for a child is much lower). But I couldn't find specific numbers for seeds.
Not being a killjoy here, I grew up around yew trees and I was always told to be careful of them, but not with any sense of panic that would suggest "any exposure at all will kill”. I think you'd have a bad time even with low exposure but death seems unlikely by accident.