How many Group Descriptor is there in GDT of Ext4?

GDT = Group Descriptor Table ; GD = Group Descriptor. I'm trying to figure out how many bytes I need to read after the superblock to extract the entire GDT, because it's needed for a program I'm making. As we know each GD is 64B, and each block group is supposed to have GD, but since there is special flags such as flex_bg or sparse_super or meta_bg which will make there less GD than there should be. In that case how am I suppose to know the length of GDT?