"How Do I Create Multiple MacOS Volumes or Partitions on an SSD or Hard Drive with APFS?"
Generally, when you have an SSD or hard drive formatted with the new Apple File System (APFS), you only want a single partition on it. You can then create multiple "volumes" within that partition. These volumes act just like partitions, except they share a single partition's space among themselves as needed. This avoids the kind of wasted space you always get with multiple drive partitions and their rigid size allocations. The problem comes when you want to place more than one copy of a recent MacOS version on the drive -- say, both High Sierra and Mojave, or even duplicates of either one for testing or easy backup. An APFS volume with MacOS installed is accompanied in that same partition by three hidden, associated volumes, including the Recovery volume. Place another MacOS volume in that partition, and your system and utilities can become confused. So, in this case, you do have to partition the drive instead of adding volumes. But on trying to, you may run into quir