Set the number of choices selected at which "{n} selected" will display. is_tax conditional left as an example.

FacetWP set n selected

/**
 * Set {n} selected text to show after 1 selection, on all facets
 */
function td_facetwp_selected() {
	if ( is_tax() ) {
	?>
	<script>
	(function($) {
	  $(function() {
		if ('undefined' !== typeof FWP) {
		  wp.hooks.addFilter('facetwp/set_options/fselect', function(opts, data) {
			  opts.numDisplayed = 1;
			  return opts;
		  });
		}
	  });
	})(jQuery);
	</script>
	<?php
	}
}
add_filter( 'wp_head', 'td_facetwp_selected', 100 );