From experience over many years, they are all standard threads and size, but not length, that varies by case manufacturer.
Generally a computer case will come with 9 standoffs to meet ATX standards, unless the case is smaller. The majority of ATX motherboards will use 6, smaller boards even less. More expensive 'wider' ATX boards will use all 9, which is nice when they are right next to the main I/O for the disk drives.
There are many cases available that use zero or only some standoffs and use threaded and stamped raised sheet metal for the stand offs.