You can choose to accept multiple answers with a Dropdown question, but you can't specify an exact number (like "at least 3 selected," "exactly 4," or "between 2 and 5"). If you would like to set a specific minimum and/or maximum number of answers, try using a Multiple Choice question instead.
Field configuration
Hover over the desired dropdown field in the editor.
Click the cogwheel icon to the right of the question to configure it.