If a centromere probe is used in fish analysis how many spots will light up in the cells of an individual with trisomy 21?

There will be three spots that will light up.

Explanation:

There is a research paper published which demonstrated the trisomy of 21 chromosome. They found out that there are total three spots that light up in Trisomy 21 (down's syndrome's fetus). I have attached picture from research paper and provided the reference below.

Reference

(Davies, A. F., Barber, L., Murer-Orlando, M., Bobrow, M., & Adinolfi, M. (1994). FISH detection of trisomy 21 in interphase by the simultaneous use of two differentially labelled cosmid contigs. Journal of medical genetics, 31(9), 679-685)

i think that is new experiments yield untrustworthy data and should be carefully considered, because you don't know what will happen with the experiments.