Singular of mitochondria, so "one mitochondrion, two mitochondria". A mitochondrion is like a small cell within a cell. Each of the cells in your body, including eggs or spem, contain their own contingent of these little energy transformation factories.
The idea that these organelles are singular, bacteria like entities is valid for some cells, but not all. It is becoming clear that in many eukaryotes they form a continuous Mitochondrial Reticulum(MR) that stretches throughout the cell. Since most images of cell histology have been from slices in the electron microscope, a false image of many separate structures was formed rather than a single unified structure. Only through 3D image reconstruction from many slices of the same cell is it possible to arrenge the components properly. Non-invasive techniques also give a more holistic picture.