Publishing in one vs. many journals
I am sure opinions and traditions vary concerning this question. For me the quality of the journal (say, impact factor) is the most important, you try to get published in good journals. In some cases, the field may be so narrow that only one or a very few journals may be good. In such cases the selection may result in a single journal. 

To publish in a variety of journals seems to be a means by itself for many but I think the reasons still vary. There may be advantages in getting published in many journals to show that the research has wider applications and is accepted more generally. It may be a way to avoid the suspicion that you have a back way into a single journal (not that that would be true). In some cases, you may select different journals because you know your ideas are not favoured by someone in a specific journal's editorial board (strategic reason).

So reasons for spreading the publications may vary quite substantially. The main point for me is, however, still that primarily the quality of the journal will decide. I would not chose a different journal just to get a new journal mentioned in my list of publications.