In general cells that need more energy, since mithocondria are responsible for making and processing ATP.
Common examples of cells with many mithocondria are muscle cells that are in current activity all the time, specially heart muscles and muslces in the respiratory system such as the diafragma since the heart beats all the time, non-stop, that is, unless you die of course and so do the muslces needed to breath.
Hope it helped,
BioTeacher101