Full list of values not showing in custom field

Summary:

My client has created a subcategory custom field in HR HelpDesk, this is to keep the list that shows to the employee succinct and when it's triaged by the agent they will then further classify the SR by selecting a subcategory from a much longer list.

Content (please ensure you mask any confidential information):

We've created a fixed choice list custom field type under the HR Help Desk Request object, created a custom lookup type with 50 items in the list but it only shows 15 in the drop down and the search does not bring back the items we can't see in the list. I can't see any limits on this lookup type in the Application Composer guide