不可数,因为财富是不可数的,在例句中也表明了这一点：He came into a large number of fortune when his rich uncle died.
其中a large number of 既可修饰可数名词,也可修饰不可数名词,而句中fortune作财富被a large number of 修饰时,并无复数形式,故可推断fortune 作财富是不可数的