It is estimated that half of all conceptions have too many or too few chromosomes. According to the text, what happens to most of the zygotes that are formed in these situations

The zygotes that have abnormal number of chromosomes present in them get spontaneously aborted. These spontaneous abortions ensures that only the zygotes with normal number of the chromosomes are allowed to mature and turn into fetus. The zygotes with more or less than the normal number of chromosomes are unstable and unable to survive for long.

Hence, the answer is 'the zygotes with too many or too few chromosomes get spontaneously aborted'.