a lot! haven't counted - many lost to neglect over winter - a couple times I have done that - but since joining OB I haven't lost any more to neglect at least
I've lost a number of Phals - I finally realized I was over-watering for the most part.
Have lost some very small seedlings - just too hard to manage moisture levels (very dry summer climate here)
A few, I have no idea why they died.
Some just die as part of the learning curve, I suppose.
In a nutshell, most have died from either neglect, or over-watering