If an organism has a diploid number of 8, how many chromatids are visible at the end of mitotic prophase

At the end of the mitotic prophase you will get 16 chromatids.

Each chromosome has 2 chromatids and, at this stage, we still don't have a division to form new cells, that's why it's 2x8 chromosomes.

In this case 2n = 8 chromosomes. (equals 4 pairs)

A haploid gamete would have four chromosomes.