Three factors enter in to answering this question.
Number 1 ). is the Catholic Church a charitable organization?
A charitable organization is a type of non-profit organization (NPO). It differs from other types of NPOs in that it centers on philanthropic goals as well as social well-being (e.g. charitable, educational, religious, or other activities serving the public interest or common good).
According to that definition yes the Catholic Church is a charitable organization.
Number 2 ) how many people make up the Catholic Church?
There are an estimated 1.2 billion Roman Catholics in the world, according to Vatican figures. More than 40% of the world's Catholics live in Latin America - but Africa has seen the biggest growth in Catholic congregations in recent years.
That means the charitable organization known as the Catholic Church is 1.2 billion people strong.
Number 3). Is there any other charitable organization that can claim more than 1.2 billion people as members?
No there is not.
Therefore the Catholic Church is in fact the largest charitable organization on earth.