At what phase of mitosis does the number of chromosomes double

Chromosomes actually duplicate in the S phase of interphase. Interphase is split into three phases, G1 phase, S phase and G2 phase. S phase is the synthesis phase, which is when DNA molecules (chromosomes) double.

Some concept fix will happen in university, because in university level, we say that chromosome never doubles, it is the amount of DNA molecule that doubles. Also, interphase is part of the cell cycle, but not mitosis (IN UNIVERSITY LEVEL, so don't worry about it)

But for high school, the answer to your question will be interphase.

Mitochondria is the cell organelle that produced the energy that is needed in the body system in form of atp, that is why it is called the power house of the cell. energy is needed by the living organisms to carry out the activities which sustain life.  if the mitochondria is destroyed atp will not be produced. if the mitochondria is destroyed, energy production will stop and the affected living organism will not have the energy to carry out the necessary life processes; this will lead to the death of the organism.