How about KoboToolbox. I have a question. How can I schedule the question to be valid only when you select 13 answers?
Welcome to the community forum. As a new user, I would recommend that you first search for your issue in the forum to see if it has been addressed or not. If it has not been addressed, then create your post using the appropriate heading/title. This will always allow you to get faster responses from the community.
On this issue, your question is not clear. However, if what you meant was to restrict the number of multiple choices to be exactly equal to 13, then what you are looking for is a constraint (see how to add constraints) and then look for previous of count-selected