if x new files are simultaneously created in the folder being monitored, does the Created event fire x times at the same time or x times one after the other?
The filesystemwatcher class uses an internal buffer to store filechanges (this can overflow, which will cause events to be lost) which will invoke events of the subscribed type one after the other in succession until the buffer is empty. This is run asynchronous by default. Default is used as long as long as the SynchronizingObject property is described in the documentation. You can make this behave synchronously by setting a SynchronizingObject as is described in this answer.
Note also, from the documentation, that only one file watcher will receive the event of a file change even if more than one is watching the same folder.
In summary: By default, these events are stored in a buffer and then fired in rapid succession by a separate thread until the buffer is empty. If many files are created or changed too fast, some changes may be lost due to buffer overflow.