Previous versions of 4D had a limit of 8 million keys per index. With 4D version 6.7, you can now store up to 16 million keys per index. Databases created prior to version 6.7 do not automatically increase the limit to 16 million keys per index when converted. Consequently their indexes will be limited to 8 million keys. To benefit from this new feature, you just need to re-index the fields with version 6.7.
Note: Do not confuse index keys with indexed records. The index keys can be used for both records and sub-records.