One way to think of the M9 is as three separate effects engines, or slots. Each engine can run one effect at a time, so you can have up to three effects active at the same time. WIthin in slot, there are two spots two put an effect, and of those two, you can have one of those active. So in total, you have six effects available to you immediately, and of those six three can be active, and you can three "on deck" to pull up easily. That particular grouping of six effects is called a scene. There are a total of 24 scenes available in the M9.

To answer your second question, you can as many instances of the same effect as you want, and you can put them in any order you want. If you wanted a scene with 3 delays running at the same time, you could do that. There aren't those sorts of limitations on the M9. You can put any of the 100+ effect models available into any of the 6 positions in a scene, and you can have multiple instances of the same effect.