How much ash does the average human body create?

depends on the heat of the fire. But it shouldn't be a lot. I'm no pyotechnician.
But consider this:
That's what a human is made of:

# Oxygen (65%) # Carbon (18%) # Hydrogen (10%) # Nitrogen (3%) # Calcium (1.5%) # Phosphorus (1.0%) # Potassium (0.35%) # Sulfur (0.25%) # Sodium (0.15%) # Magnesium (0.05%) # Copper, Zinc, Selenium, Molybdenum, Fluorine, Chlorine, Iodine, Manganese, Cobalt, Iron (0.70%) # Lithium, Strontium, Aluminum, Silicon, Lead, Vanadium, Arsenic, Bromine (trace amounts) Reference: H. A. Harper, V. W. Rodwell, P. A. Mayes, Review of Physiological Chemistry, 16th ed., Lange Medical Publications, Los Altos, California 1977.

<---- most of that burns perfectely well. I would guess (guess!) that about 1lb of ash will remain. or nothing, if the fire is hot enough...