This is a reference to Ibn ʿAbd al-Barr's estimation that twelve thousand false ḥadīths were fabricated and circulated among the people. Hossein Modarressi cites this reference in his discussion of some of the features of al-Ṣādiq's views on ḥadīth fabrication.
This source is part of the Online Companion to Hossein Modarressi, Text and Interpretation: Imam Jaʿfar al-Ṣādiq and His Legacy in Islamic Law (Harvard University Press, 2022).