Hi ​@marta.podsiad I think you could use the StatisticsCalculator. GroupBy ObjectID and that should give you counts for attachments when selected in the Statistics to Summarize.

Hope that helps!

Assuming the following dataset : 

One way to do that could be the following

AttachementSum conditional value : 

GroupID conditional value : 

This method will create groups based on the attachmentSum. Each time the attachmentSum exceeds 1000, the groupID is incremented and the attachmentSum is reset.

It’s a bit tricky, and workbench will give alerts about non-existing attributes, but it will work : 

You can then use the groupID attribute to create lists of obectIDs, concatenate, etc…

Hope that helps !

It’s a bit tricky, and workbench will give alerts about non-existing attributes, but it will work :

Tips: In a similar case, I would expose the new attribute names with AttributeExposer beforehand, in order to avoid the alerts.