From LimeSurvey Manual
Maximum answers (max_answers)
It can be used to limit the total number of answers a user can select in a multiple choice question.
- Any number value
- Any SGQA code
- Any question code (variable name)
- Any equation, using ExpressionScript syntax, without surrounding curly braces.
If set to 2, the user will only be able to check at most two checkboxes or one checkbox + fill the "Other" field (if any). For the Ranking question type, the participant would be able to select maximum two items.