List fields on a Contact Form can have one of three input options:
Single Choice
Limited Multiple Choice
Unlimited Multiple Choice
How to set Multiple Choice options
https://www.loom.com/share/c49b1db5df4a40b6abda3415f280525c
Go to the Manage Contact Forms page
Select Edit for the Contact Form you need to edit
Press the number next to the field
It launches the popup to choose the Multiple Choice inputs
Limited Multiple Choice lets you decide how many options a user can choose
Press submit to save and close the popup
Press Submit to save the updated Contact Form