Complex sentence: The picture, which was Van Gogh's last self-portrait, was painted in September that year, shortly before he left Saint-Rémy-de-Provence; it was one of 43 he had produced in the previous ten years.
Split into simpler sentences: The picture, which was Van Gogh's last self-portrait, was painted in September that year, shortly before he left Saint-Rémy-de-Provence. This self-portrait was one of 43 van Gogh had produced in the previous ten years.