On this page, Apple mentions a limit of 15 view-through impressions per source app:
This means for a given source (publisher) app, iOS will only store 15 SKAdNetwork impressions at a time. That could mean 15 impressions for the same advertised app, or one each for 15 different advertised apps.
SKAdNetwork view-through impressions “expire” after 24 hours if they don’t lead to an install. But, the oldest one will expire before the 24 hours if the source app serves the user a 16th impression. This could present a problem for a high-density ad implementation (i.e., if a game shows view-through ads for 16 different apps over the course of a 1-hour play session, the first touch point will be discarded).
It's not clear why Apple chose this limit, other than perhaps for technical simplicity.
Updated almost 2 years ago