I only want 20 entries for each of the items listed in the drop down me

I only want 20 entries for each of the items listed in the drop down menu field. Each time an item has reached 20 entries, I want it to be removed from the list.

That is not possible in regular drop down fields. You can try using the following widget:

1) https://widgets.jotform.com/widget/ticket_purchases 

2) https://widgets.jotform.com/widget/inventory 

Let us know if you need more help.