The verb comes second, indeed, but it is not (always) the second word in a sentence. The word order would (normally/most often) be subject - verb - object. This sentence doesn't have an object, and the subject is achttien katten. So, achttien katten comes first, and afterwards comes the verb: spelen.
Because that translates to achttien katten spelend (not a full sentence), and not to achttien katten spelen (full sentence).
It's the same difference that you van find between 'eighteen cats playing' (not a full sentence) and 'eighteen cats are playing' (full sentence).
Hope this helps.