They say a cluster is a small group or bunch of something. Fair enough. I guess asking how big a small sample is would be like asking how long a piece of rope is: it depends.
Despite its imprecision (or perhaps just because of it) the word is very useful and versatile. You can create the verb to cluster from it, you can spot trends when you detect a cluster in your data, you can give a name to a group (or bunch) of things you would otherwise just call… er… a group of things. Yes, a cluster is much better.
That is probably the reason German has adopted the word despite having a whole lot of synonyms for it (Gruppe, Traube, Bündel, Schwarm, Haufen…). Because it is not exactly the same.
But what about Spanish? I like cúmulo, but it is not so versatile. They also use racimo and grupo, of course, but that sounds weird in most contexts. Sometimes Spanish even allows for clúster, that would be the best solution. Only that I generally have the feeling that saying clúster is not a proper translation and I am not sure my clients understand me. But as long I do not find something better, a clúster it will have to be. Unless somebody finds a better translation.