Currently the "Combine a group of fields" has a 15 data source limit.
The group by provides a way of keeping together fields in unification, like address fields, that should not be treated as individual fields, The current limit means that you would need to pick which data sources not to combine and show in the end unification which is problematic.
Please can this be removed so it can match the number of data sources.
Comments
details on the "Combine a group of fields" can be found here : https://learn.microsoft.com/en-us/dynamics365/customer-insights/data/data-unification-merge-tables
Category: Data Unification