Well, $"ionization energy"$ is the energy required for the formation of one mole of cations and one mole of electrons from one mole of starting element:
$M(g)+Delta_"ionization energy"rarrM^(+)(g) + e^(-)$
Both products and reactants are specified to be in the gas phase.
$"Ionization number"$ is a bit more old-fashioned, and you must be looking in an old text, and it is the number of that the element possesses, OR the number of electrons that the element must gain to reach a Noble Gas configuration. Usually this follows the $"Group number"$; certainly this is true for metals, but for $"Groups 15, 16, 17"$ usually ionization numbers of $-3,-2, and -1$ would be commonly reported.
