Also I think, if you increase the width of the gutterpart, then it will paint more marks.
Yeah, I see with Lazarus bookmarks + breakpoint that it should work but here it doesn't, although the code shows well that there's a paint rect moved to the right for each icon on a line (TSynGutterMarks.PaintMarks).
The gutterpart doesnt autosize, but that does not happen with 2 individual parts either.
In the IDE, the width is for 2 marks.
There is a trick to get more items per line in the IDE, put bookmarks into several lines, then join the lines. And then max 2 are shown. (That is expected behaviour, clean cut off, if does not fit.)
But if you set the width bigger by hand, then they should show... (you need to know, the max count of items on one line)
Well if not then it sounds like a bug, and needs a reproducible case.