When you load a .gltf file (the JSON manifest), you can’t know how many files are referenced by the asset at the beginning of the loading process. Consider to convert your glTF to a glb in order to avoid this situation.
BTW: When using one of the progress/loading indicators from this post, it’s not necessary to know the total file count in advance. The indicators don’t need the information about the total file count in order to provide an appropriate visual feedback.