Answer: Researching the internet, the count of oil paintings in St. Peter's Basilica was indicated to be 1. Watching a Rick Steve's travel film, it was stated the painting count was zero. The images are tiled mosaics, not oil paintings. So, if anybody has the absolute proof-positive answer, let us know.

In the 17th century, oil paintings were replaced by mosaic tile images in St. Peter's Basilica. Since 1727 there has been a Vatican laboratory dealing with tile image technology.

See:
 • James C. Hamilton, Mosaics on Vatican City Stamps
Vatican Notes, Volume 65, Issue: 371 2017, Pages 12-29


Our Lady Of The Vatican Stamps
Are Examples of Mosaic Tile Images On Stamps
Scott #1205, 1209, 1213 (2002)