There is a set of solutions here but did you consider to edit your ad's (is it a graphic file?) metadata content?
If each of yours ad would have proper string in a metadata field, i.e. "description" => InDesign text variable would display it whereever you place (rearrange) the file/s.
Using this, only you need is take care about proper file metadata content.
Each ad is a graphic, specifically a PDF. I could edit the metadata, but the placement of the incremented number isn't the same for each one. Some are centered just below, some are superimposed over the graphic.
I have actually made a lot of progress using paragraph numbering. Applying a style with "Circle # ^# on the reader service card" as the number style gives me:
Circle # 1 on the reader service card
Circle # 2 on the reader service card
Circle # 3 on the reader service card
--incremented on each Text Frame the style is applied to. This actually solves half my problem fairly simply.
The next part is incremental numbers within a frame:
IBM ibm.com - 1
Costco costco.com - 2
Apple apple.com - 3
In this case I need the number to appear at the end of a paragraph, which doesn't seem to be supported within styles. Is there nothing like a programmable field in InDesign, where I could insert:
IBM ibm.com - <<count>>
Costco costco.com - <<count>>
Apple apple.com - <<count>>
--and have a script return an incremented number each time?