I want to change all "autonumbering" of sentences in my document that have about 1000 pages to an objecct e.g. a triangle, that contains the same number, as seen above.
Not easy, especially if you want to keep the automation... This could probably be scripted, but before going into this deep process, you may consider editing your font.
Here's a example using Fontforge with a custom shape...
The problem is obviously that it would only work for lists up to 9 and not more. Maybe building ligatures would be possible, but I'm not familiar enough with Fontforge...
Another possibility is to number the list automatically in ID, then convert the numbers to text.
Then make the resulting list (including the numbers as text) into an automatic bulleted list. Use a font with the desired open triangle as a bullet character.
Now, with nothing after the bullet character (i.e.no tab or space) track the text numbering back inside the bullet character. You may have to reduce the type size of the numbering a little.
I tried this with an open circle as bullet character and it worked. I had to fiddle with the baseline shift of the bullet character (which can be controlled by a character style) but it worked.
In summary, make a numbered list, convert to text, and apply automatic bullets with a suitable shape for a bullet, the use tracking, baseline shift and type size to get the effect you want.
Seems messy on the whole, so maybe you can design yourself out of the requirement. Good luck.
If you can find a font that includes all of its numbers inside a triangle shape, you can use that font to create a specially formatted numbered list that uses that font for the numbers.
The other possibility is to insert a triangle special character before the numbers in your numbered list (this article shows you how to do this by combining numbered lists and a table of contents list), and then experimenting with the tracking and kerning options for those characters (using those options and negative numbers) until your numbers appear to be located at the same location as your special character.
I don't know for sure that this would work, but you could try playing with it.