Is there one Islam or many Islams

It is not immediately clear whether there is one Islam or many Islams. If it is singular then the task is to identify which of the many competing claims to adopt as true Islam. By singularity of Islam I do not mean singularirty of interpretation. Of course there can be room for differences within Islam. But my questions is, is or are there fundamental elements of Islam that are not up for interpretation or where a difference of opinion simply cannot occur without rendering Islam a plural religion. By plural Islam I mean that any Muslim can define Islam as they see fit.