it really helps to look at an actual disc in a hex editor. but basically you read it like this
1) get the total # of partitions from 0x40000
2) get the offset of the first partition table
3) jump to that address ( usually 0x20 bytes after 0x40000 )
4) read 4 bytes. if you get something other than (u32) 0x00000000, read 4 more bytes. keep repeating this step till you dont get any more partitions.
5) you have just read the first partition table. if the total # of partitions you got in step 1 is higher than all the partitions you found already, go back to step 2 and read the next 4 bytes ( next partition table offset )
really every official game will have 1 partition table except SSBB. multiboot discs made with wiiscrubber can have all sorts of different layouts.