about special relativity, relativity of mass

I am a starter. If there are some problems about this passage, please forgive me.
I've studied relativity in these days. I make a confusion: why is mass relative?
我記得質量是指物質的量啊!
i.e. If the number of atoms of a matter remains unchanged, the mass of it is unchanged too.
So why is mass relative?

What exactly have you studied---what is your current understanding about special relativity?

To put it simply, mass could become "relative" because in terms of measurement it varies with the observer's frame of reference i.e. the observed speed of the measured object. This concept is called "relative mass".

The object also has another property---it's mass when measured at rest i.e. observed speed=0. This property is invariant as long as "atoms of a matter remains unchanged". Therefore it's called "rest mass" or "invariant mass".

You'll have to realize that "mass" is not a single solid concept in modern physics. It can be defined differently, representing different physical properties. That's why we need a prefix to clarify it's exact meaning. What you described "If the number of atoms of a matter remains unchanged, the mass of it is unchanged too" is referring to the concept of invariant mass, which indeed is not relative (remains the same to all frames of reference). However, if we actually measures the mass of a MOVING object, the measurement itself varies with the object speed, thus this measurement is relative, and it's called "relative mass".

An atom or a bunch of atoms can have different value in "relative mass" if it is measured in different frames of reference, however, it can only have one "invariant mass", this is a fixed property (ignoring subatomic changes).

o~ thx
I think that I'm too fixed in Newton's definition.
So if an object is moving, each atom of the object is changed in mass, isn't it?

As I have already mentioned, this question is ambiguous because you didn't specify which mass property (relative or invariant) you're talking about. So, the answer could be either yes or no.