The :ls command will list the buffers you have open. What vim doesn’t tell you though is that there are some unlisted buffers that it isn’t displaying. To see all of the buffers, you can use :ls!. According to the vim help file:

When the [!] is included the list will show unlisted buffers (the term “unlisted” is a bit confusing then…).

This reveals buffers for netrw, :help files, etc. This helps explain the sometimes sporadic numbering that vim uses for buffers.