Case 1: The main kernel creates N grid kernels for a total of N+1 kernels. The main kernel is not taking part in the grid computation, so it is idle, waiting for the grid computation to end. Thus the main kernel does not require significant cpu time, and so each grid kernel gets exclusive access to a CPU. N cpus, N running kernels.
As @Carl Love says, the main kernel will be swapped out, so it will not interfere with the running kernels. If you look at a process monitor you should see N+1 kernels, but only N of them actively running. If you run the simple example I posted near the top of this thread you should see that behaviour. Now, if you look at the process monitor, you'll see that there are dozens if not hundreds of processes on your system, the vast majority of which are not actively running. These idle processes are not assigned a specific CPU. When they need to run, the OS will put them onto a CPU when one is available, and move whatever process was there off. However this happens very quickly so that if there is a processes that wants to run at 100%, you'll rarely notice the other processes getting swapped in and out.
Case 2: The distinction between case 1 and case 2 is if the kernel calling Map is the main kernel or a grid kernel. Main kernel is case 1, grid kernel is case 2. For case 2 to happen, Map called by a grid kernel, there must already be grid kernels, so Maple will not create more grid kernels in Case 2. Some other command, for example Launch, was called and that call created the grid and launched a computation. As part of that computation the grid kernels call Map, which is case 2. Case 2 happens on an existing grid, it does not create one.
When Maple creates a local grid, it follows the same default behaviour no matter which Grid command is used to create it. Maple will create N grid kernels (for a total of N+1 kernels).