How many chromatids are there in a duplicated chromosome

In a duplicated chromosome, there are two sister chromatids. 

During the S phase of the cell cycle, which precedes cell division, each chromosome replicates to produce two identical copies of itself. These two identical copies, called sister chromatids, are held together at a region called the centromere. 

The two sister chromatids contain identical DNA sequences because they are produced by the replication of a single chromosome. They are attached at the centromere until they are separated during cell division. 

Once separated, each sister chromatid becomes an individual chromosome and is distributed to daughter cells during cell division. Therefore, a duplicated chromosome consists of two sister chromatids, while an unduplicated chromosome consists of a single chromatid.