Hi @Clara,
That's an important question but unfortunately there isn't a very clear and easy answer as it depends on many factors. I made an attempt at highlighting some of the factors on another thread here which might help you decide. That being said, I think for most common samples 6,000 would be sufficient coverage. What is the sample type being analyzed? I personally would drop the last 2 samples (791, 78) and rarefy at 6,202.