Limited dropdown choice

Hi.
I’m testing the plugin to show family relationships within the members of the site and as such the dropdown menu for a field will have to be populated by the entire membership (around 1300). Not all the members can be seen nor searched in the dropdown menu on testing.
Can this be fixed or is there really a limit to the number of possible choices for the dropdown?

Thanks in advance.

https://wordpress.org/plugins/um-relational-fields/