There is no scientific evidence that any person ever spontaneously combusted, and no scientifically known means by which it could happen.
In a typical case where there has been a report of a body burning unusually, a simple or at least plausible explanation is available when the case is examined in detail.
A statistically typical case might involve an elderly or infirm woman who is alone and near a fire source at night and in winter. The fire typically starts on or near the person, and it often involves the clothing, chair, or bed.
In all cases there are fuel sources, a plausible explanation for the ignition, and sufficient oxygen.
The idea that the body could spontaneously ignite was promoted by the nineteenth-century temperance movement. Leaders of that movement correctly noted that many fatal burning cases involved drinkers.