Yeah, it just comes with one, although the spec on the Elektron M:S page says that two come in the box. 
As avantronica said, there’s two different spec midi to minijack adapters - basically both are wired in the opposite way to eachother.
The MIDI In port is will automatically configure itself for either standard, so any adapter should work with it without any configuration. The MIDI Out port needs to be configured in the Settings to use one or the other polarity. Here’s the bit from the manual.
Obviously using the official Elektron ones negates the need to change the configuration, but they are expensive. There are other third party ones that are cheaper that use the same polarity as the Elektron ones, but I don’t know specifically which ones, so I can’t point you directly at them. As to what to do with the spare one if you buy the set of two - I guess keep it for when the dog chews one of the ones you’re using.