Mitosis is a complex mechanism that, through mutations and crossover, determines how a chromosome is composed. But at that point, the number of chromosomes is already determined: how can this number evolve through generations? What mechanism is involved?
A mutation is a general term referring to any alteration of a nucleotidic sequence. It includes small mutations like indels (insertion or deletion of a single nucleotide) and substitutions, or bigger mutations such as e.g. chromosome fusions, whole chromosome duplications, Robertsonian translocation, whole genome duplications and gene deletions. Some of these bigger mutations can affect the number of chromosomes (and ploidy number).
Such of these mutations can typically be caused by the non-disjunction of homologous chromosomes during meiosis.
Most of these mutations are deleterious but some aren't and might eventually fix (fixation = reach a frequency of 1.0) in the population.
A very common example of such mutations in humans is the mutation that is causing the Down syndrome.