Why is an entity reference view field only showing one view result?
I was having this issue as well, and it came down to the view returning a different entity_id than the one(s) I thought it should be returning. This happens because of incorrectly formed relationships.

A way to check this is to add a "dpm($options);" (if you're using the devel module) in the entityreference_options_list function of entityreference/entityreference.module file (just after the $options are returned from entityreference_get_selection_handler) to see what options it is retrieving from the view.

I started a new Entity Reference view that just returned the entities that I wanted, and then I built my relationships backwards from there to finally get the correct number of entities in my form.

I hope this helps!