For templates like w:Template:Multiple image and w:Template:Maplink, there can be an arbitrary number of parameters for images in the collage or points on the map. TemplateData does not currently seem to have a way to handle this; the best that can be done is to add TemplateData manually up to a dozen or so, which covers most use cases. However, this is tedious, first of all, and it clutters the documentation by creating a bunch of repetition. There are a lot of templates like this, so I'd like to see this fixed at some point.
Topic on Extension talk:TemplateData
Yes, this is a known limitation, see phab:T54582. I'm afraid there are currently no plans to work on this.