In the chapter `Cell cycle and cell division` in NCERT, it is given that during S phase, the DNA per cell doubles but chromosome number remains the same. How is this possible? Are they both related? Why the increase in DNA doesn`t change chromosome eventhough DNA is a part of chromosome??!!

During S phase, which follows G1phase, all of the chromosomes are replicated. Earlier they had single chromatid. Following replication, each chromosome now consists of two sisterchromatids. Thus, the amount of DNA in the cell doubles, even though the chromosome count, of the cell remains at 2n
Note: DNA content from 2c becomes 4c after replication but the number of centromeres and chromosomes remains unchanged. In picture left and right both will be considered as a single chromosome.
Remember during Anaphase only when the centromere will break and the sister chromatids will separate at that time each chromatid will be considered as a chromosome.