We need to be able to identify if we reach the 250 values limit or not.
Adobe Client Care answer:
The limit of 250 values is saved at a visitor by visitor level in the back end of the data base. This information is not available for which visitor has the value reached 250 and for whom it has not. The report for list variable shows aggregate values.
Further, impressions may not be exactly same in case user had got some values expired because of reaching the limit of 250 values already saved for that visitor and only the latest value could be sent to reporting.