The number of chromosomes is fixed. We did not slowly gain additional chromosomes in an evolutionary process.
Actually, that's not true either. I should have said, in most cases. Actually, the number of chromosomes in an organism can vary both numerically and by structure. However, this happens much less frequently than gene permutations and these tend to be more severe mutations. Very, very few are beneficial, many are neutral, some are catastrophic. But they do occur and are transmitted genetically,
As an organism becomes more complex, the number of potential variations increase, but those variations become more subtle.