I have a list with a choice field, that has 20 different choices in it. Once three new items have been added that select the same choice, I want that choice to be eliminated from the list (or throw a customizable error to select another choice).
Is this possible?
If my explanation wasn't clear:
Let's say my choices are Red shirts, blue shirts, and yellow shirts. I only have three of each. If three people fill out the form and they all select Red shirts, then Red shirts shouldn't be an option for the fourth person to fill out the form (or should throw an error).
If I understand your requirement correctly this is more about checking inventory if the particular product "red" is available or not. This is similar to any product quantity check i.e. you may have to design the solution in such a way that you may need to store the inventory list and the quantity available in a different list and when a user saves after selecting the record you may have to reduce the inventory by 1.
You should not check this before the form is saved (like in the scenario you mentioned) i.e if 3 people open the form at the same time and select "red" the 4 guy should also be able to atleast select as you are not sure if those 3 people will save their changes or just cancel and come out of the form?
You can do all the validation in JS or have a calculated field to do a lookup on the "available stock" and if the "avaiable stock" is 0 then throw a validation rule error or a custom JS error to show the user if they select that stock.