make up one’s mind（作出决定，拿定主意）和change one’s mind（改变决定或主意），当它们的主语是复数时，mind 原则上用复数。如：

1. make up one’s mind（作出决定，拿定主意）

Every day I’d ring up home and asked if they’d changed their minds.

She asked the students to explain why they had sometimes changed their minds about a correct choice they had made.

2. change one’s mind（改变决定或主意）

There's no percentage in arguing with them. They'll never change their minds.

Christopher hoped to exert his influence to make them change their minds.

Friends tried to persuade them to change their minds, but neither man would listen to reason.

但偶尔也有主语为复数而 mind 用单数的例子，不算普通：

They decided to change their mind. 他们决定改变主意。（薄冰英语语法）

They find it difficult to make up their mind(s). 他们难以下决心。（新时代英语惯用法词典）