I've asked this before but got no answer.
In particular are there any other sources other than Musashi's own account. It seems like a pretty bold claim. Also What would have been considered a duel in Musashi's time and era? Would it have been to the death? It seems like it would be difficult to learn anything without ever making a mistake.
According to William Scott Wilson's biography The Lone Samurai, Musashi did have around 60 confirmed duels, starting with his defeat of Arima Kihei at age 12 (Stated as 13 in The Book of Five Rings, as infants were considered to be already 1 year old at birth in Japan at the time). That being said, much of Wilson's book is pieced together from varying sources, some conflicting. It isn't unreasonable to say the number may well be inflated.
As far as what constitutes a victory changed for each duel, some were fought to the death, and others mere tests of skill against one another. Musashi was famous for often fighting with a bokken (a wooden practice katana), even against live swords. Some of these fights still ended in the deaths of his opponent, perhaps most famously Sasaki Kojiro, the Demon of the Western Provinces. Legend states that Musashi carved a boat oar to a bokken while in route to the duel, where he quickly struck down his opponent, and finished him off by caving in his chest. After this duel, Musashi's status as a swordsman was solidified, and he stopped finishing his duels with such extreme prejudice. From about 30 onward, he stopped killing his opponents, instead proving his superior skill by not allowing his opponent to make any ground, and generally out moving them. Most opponents quit in frustration. This continued into his late fifties. Considering he started at age 12 with his defeat of Arima Kihei, and the variety of duels fought over his life, I personally believe the claim. I'd highly suggest reading Wilson's biography. It's considered one of the most complete and accurate biographies available on Musashi.
One possible source is The Book of Five Rings itself, in which Musashi states (pg 4), "Although I dueled more than sixty times, not once did I lose." His words, written in 1643, translated by Thomas Cleary in 1993.
Side question. How romanticized are samurai and bushido? The only thing I've ever read is the Hagakure but I'm pretty sure that was written during the Meiji Era when there weren't any samurai.
But wasn't Zatoichi a more accomplished duelist? Many of his duels are canonized on film.