If you only want certain options (or answer choices) can only be selected a certain number of times, you can enable Inventory Control for certain options.
Once logged in, you'll go to your Published Forms.
Select the Form Menu for the Form that has the question(s) you want to add inventory to.
Select Edit Form.
Select Add/Edit Questions and Answers
Select the Question you want to add inventory to and then Review/Edit Question
Select the edit icon under the Inventory Control section.
Change the status to Enabled, enter how many times the answer can be chosen, enter what you want the memo to read once sold out and then you can decide if you want to have the number of spots available display to registrants (by checking the Display Inventory Counts publicly box) and select Save.
Once saved, you can see the settings you've just enabled.
Once an option is sold out, it'll gray out the quantity field on the form and display your memo.