Why are there so many or so few fossils for any given organism?

Organisms decompose more quickly when they are in contact with oxygen. ... When an organism is buried quickly, there is less decay and the better the chance for it to be preserved. The hard parts of organisms, such as bones, shells, and teeth have a better chance of becoming fossils than do softer parts. Got the answer from https://www.americangeosciences.org/education/k5geosource/.../fossils/how-fossils-form
Hope it helped