If there are 30 chromosome in G 1 phase then what will be the no of bivalent in zygotene phase?

It will be 15 bivalents or Tetrads.

During S-phase the 30 chromosomes will replicate but they will be attached at the centromeric site by cohesin molecules and hence the number chromosomes will remain 30.

When zygotene stage begins, the homologous chromosomes pair up to for bivalents. Hence there will be 15 bivalents, 30 chromosomes and 60 chromatids.