How does chromosome number get halved in meiosis I?
If you consider sister chromatids to each be a chromosome, meiosis I, rather than halving the chromosome count, separates the homologous chromosomes, which result from sexual reproduction, from one another. Meiosis II then separates the sister chromatids from one another resulting in the final halving of chromosomes.

On the other hand, many if not most biologists don't consider the sister chromatids to be individual chromosomes until they are segregated during meiosis II. In this sense, meiosis I does reduce the chromosome count to half.