I have referred this to the plugin developer, Ross, for his input.

Hi Maurino

We have a WP / PHP hook which you can use to modify each individual option in your field – you could then loop through all your options, count the total results, and add that to the first option (All Items) – this is the filter: https://searchandfilter.com/documentation/action-filter-reference/#filter-input-object

I’ve actually gone ahead and managed to create a code snippet for you that does this:

function add_count_to_first_option( $input_object, $sfid ) {

	// if the field doesn't have input options, then leave it alone
	if ( ! isset( $input_object['options'] ) ) {
		return $input_object;		
	}

	$total_count = 0;

	// loop through each option and add up the total count
	foreach ( $input_object['options'] as $option ) {
		if ( isset( $option->count ) ) {
			
			$total_count += absint( $option->count );
			
		}
	}

	// now check to make sure there was a first option (with value of "")
	// and update its count
	if ( isset(  $input_object['options'][0] ) ) {
		if ( '' === $input_object['options'][0]->value ) {
			$input_object['options'][0]->label .= ' (' . $total_count . ')';
		}
	}

	return $input_object;
}
add_filter('sf_input_object_pre', 'add_count_to_first_option', 10, 2);

Add this to your themes functions.php, and that will add a count to all first options in your fields.

Thanks