Here is a summary: you already know that a diploid cell (2n) stays diploid in mitosis. How can 1 diploid cell make 2 diploid cells? During Anaphase each split chromatid counts as a chromosome because dipoid cells undergo replication (hapoid cells don't) during interphase(= 4n).
For Meiosis, you should already know it creates a hapoid from a diploid. Meiosis I creates the haploid by splitting the sister chromosomes (not chromatids). Meiosis II for one of those haploid cells undergoes "further meiosis" and each chromosome splits into two chromatids, each one going to the new cell.
So it goes 1 cell with 2 sister chromosomes => 2 cells with 1 chromosome each => 4 cells each with 1 chromatid
