JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
I would suggest to use radio button instead of checkbox if you are requiring user to select only one option. Radio button options already limited to single selection every-time.
In Check boxes also, it is possible, to limit the maximum or minimum required number of options. For doing this, right click the check box field, show properties then you can see these options.
You can also check the below guide, which explains the same thing in more detail.