I could not find this option in the admin panel (and no plugin for that), so I used jquery to not let the user send his question if there are NO tags.
use this jquery code:
// user must give at least 2 tags
if ($("#tags").length > 0){
var regForm = $(".qa-form-tall-table").parent();
regForm.submit(function(e) {
var tagsN = 0;
// count nr of tags
var matches = $("#tags").val().match(/\b/g);
if(matches) { tagsN = matches.length*0.5; }
if(tagsN<2) {
e.preventDefault();
$("#tags").focus();
$("#tags").css("background-color","#FFFFAA");
alert("Please give 2 tags related to your question!");
return false;
}
return true;
});
}