That is not possible at the server level but you should be able to use JavaScript and Expression Manager to control the display of the submit button.
Given question codes q1-q50, you could add something like this to the source of a text display question on the last page of the survey (replacing "q1, q2, q3...q50" with all of your question codes):
<script type="text/javascript" charset="utf-8">
$(document).ready(function(){
$('#movesubmitbtn').hide();
if({count(q1, q2, q3...q50)} > 20) {
$('#movesubmitbtn').show();
}
});
</script>
