Complex sentence: A group of scholars estimate that there are only 300 completed works by Hitler over the span of his life; however, Hitler mentioned in his book, "Mein Kampf", that while in Vienna, he produced around two or three paintings a day.
Split into simpler sentences: A group of scholars estimate that there are only 300 completed works by Hitler over the span of his life. But Adolf Hitler mentioned in his book "Mein Kampf" that while in Vienna he produced around two or three paintings a day.