你买了多少蛋糕用英语可以说成 "How many cakes did you buy?"。来源是因为在英语中，我们使用动词 "buy" 来表示买东西，"many" 表示多少，"cake" 是指蛋糕，而这个句子的主语是你，所以我们用了动词的过去式来表示你已经买了东西。
例句：
How many cakes did you buy? (你买了多少蛋糕？)
I bought five cakes. (我买了五个蛋糕。)
She bought a cake for her friend's birthday. (她为她朋友的生日买了一个蛋糕。)