Consider the average adult who weighs about 70 kg. They are about 18% carbon which means that they contain about 12.6 kg of carbon.
The molecular weight of carbon is 12.01 grams per mole. Thus an average person contains approximately 1049.13 moles or 6.32 x 1026 atoms of carbon. That’s 632 with 23 zeros after it.
When a person is cremated, almost all the carbon from their body is burnt (that is combined with oxygen) and is released into the atmosphere as carbon dioxide. This joins the carbon cycle and can be absorbed by plants which add the carbon into their tissue and release the oxygen. Plants are eaten and become part of the food chain up to and including humans. (Just think where the carbon in that steak and peas came from).
The bodies of people who are buried eventually break down and join the same cycles. Thus atoms from every person who ever died are being recycled.
Much of the cycling carbon on Earth is involved in the biomass. The total biomass (all living and once-living matter) weighs about 1,877.29 billion tonnes.
There are about 6.5 billion people on Earth. Many of these are children, so for the sake of approximation, lets assume the average weight of a human to be 50 kg. Thus people make up around 325 million tonnes or 0.017% of the biomass.
So if carbon atoms from a departed human are split proportionally throughout the biomass, there should be around 1.07 x 1022 atoms shared amongst humans. Assuming this spread is uniform, then that gives a staggering 1.65 x 1012 atoms of carbon per person.
This means that you may contain around 1650 billion carbon atoms from any previously deceased person.
Of course carbon atoms will have passed through numerous people both living (from carbon dioxide exhalation) and dead.
But think of every historical figures you admire. Chances are they are now part of you!
Values and proportions except where estimations are explicitly described are all taken from Wikipedia.
All billions are given as US billions ie. 1000 million.