How much is too much when it comes to diagrams in a research article?
That entirely depends on the article itself. In some cases diagrams aid an explanation and make it clearer. In others they offer little insight. If an article depends on having many diagrams of the former kind, then I don't see that "too many" is an issue. But even a few of the latter kind may just get in the way of understanding. 

Words and pictures can be complementary. It is a judgement call by the authors as to which is best in a given case. They may err a bit, but that is human nature. 

In general, think about whether words or pictures/figures better convey your meaning and act accordingly. Sometimes both are needed. Examples illustrating (note the word) a concept are often best presented in figures. Imagine a Calculus textbook without figures. Now imagine one without formulae. Would either work?