How many generations between the first lifeform and humans are there?
The generations that preceded multicellular life will so vastly dominate the answer that the last 500,000,000 years don't even matter. So life is 3.5 billion years old, and for most of this time, life was prokaryotic, dividing every 20 minutes or so. Thats 26,000 generations per year. Multiply that by 3.5 billion and you get 9 trillion generations. That is likely an overestimate, based on optimal growing conditions of bacterial in exponential growth. But it is unlikely to be orders of magnitude off.