Yes, you can specify a minimum number of choices for a multiple choice field. Go to the Editor of your online form and click on the Multiple choice field located in the main view.
In the Edit Field panel on the left-side, under the Validation section, you’ll find the Min no. of choices option.
If you want your form users to select a specific number of choices (e.g. exactly 2 choices), you’ll need to specify the exact value for both the maximum and the minimum number of choices. Make sure the value typed in for the Min no. of choices doesn’t exceed the value you have for your Max no. of choices, otherwise you’ll receive an error.
When your form users don’t select the minimum number of choices you’ve set up, they will receive an error message. You can change the text of the message in the Translations → System Messages section.