In an onion cell nucleus,there are normally 12 chromosomes,in which stages of mitosis are there 24 chromosomes

I thought it would be Interphase or Prophase (before the cell splits and therefore when there is twice as many chromosomes) but the book says its Anaphase and Telophase?
Please explain

There are normally 12 chromosomes, each consisting of a single 'chromatid'.

In the S phase of interphase, the chromosomes are duplicated. But the two genetically identical copies remain bound together. There are still only 12 chromosomes, but now each one consists of the 2 chromatids.

Chromosomes numbers don't change in G2, prophase, or metaphse: in all of them there are still 12 chromosomes, each consisting of 2 sister chromatids.

In anaphase, the sister chromatids separate. The instant they do, each one becomes a full fledge chromosome. So what was a single chromosome consisting of 2 chromatids, becomes 2 chromosomes, each consisting of 1 "chromatid". Thus the number of chromosomes has doubled ... there are now 24 chromosomes in the cell.

In telophase, the cell still has not divided so there are still 24 chromosomes in the cell, even though they are at opposite poles with different nuclear envelopes forming around them.

After cytokinesis, there are two cells. Then each one again has 12 chromosomes, each consisting of a single "chromatid". We are back where we started.

Yes.