The smallness of his oeuvre speaks for itself. There are fewer than 20 generally accepted autograph works and some of those are unfinished. Of course there's been some attrition- but not much. There was a painting of Leda and the Swan which a royal mistress is supposed to have ordered to be burned because she thought it indecent. We know it from copies. Do the copies count as authentic Leonardos? Apparently not. But they came out of his workshop and he must have exerted some degree of quality control. He probably had as much to do with them as modern artists like Koons and Hirst do with the product of their art factories. Here's an oddity: the autograph touch is valued in an old master but not in a modern one. Hirst has assistants mass producing his spot paintings- and yet they sell and are valued as genuine.
The shortage of authentic Leonardos is an affront to the contemporary art market- which keeps digging up new ones. A few years back there was a pretty chalk drawing called La Bella Principessa which didn't look much like a Leonardo and is almost certainly a fake. (Bolton-born forger Shaun Greenhalgh said it was one of his and he'd based it on a girl who worked down the local Co-op). More recently there was a grotty drawing of St Sebastian- which could be genuine but is hardly a major work of art, and- again not a major work of art- there's the Mona Vanna- a charcoal drawing of a nude woman in the Mona Lisa pose which has traditionally been ascribed to a pupil but is now being upgraded. Most famously there's the oil painting Salvator Mundi which changed hands for an enormous sum and has since disappeared because it's almost certainly not what it was sold as but a horribly bashed about studio production that has been restored (meaning heavily repainted) into a condition of smooth perfection.
The one painting that Leonardo never got bored with is the Mona Lisa. He kept it by him through several moves and never seems to have stopped tinkering. It may have started off as a portrait of an identifiable Florentine lady but it ended up as something rather different. What it is exactly is anybody's guess- hence the fascination. I think it's a picture of his soul.