Use this code to have a field in your form that has a counter for each user. It works similarly to the [auto_id] shortcode, but each user has their own count, starting with 1 and increasing for each entry they submit.

Change 2806 to the counter field in your form.

add_filter('frm_validate_field_entry', 'add_user_entry_counter', 10, 3);

function add_user_entry_counter($errors, $posted_field, $posted_value){
	$field_id = 2806; // Change 2806 to the ID of the user entry counter field.
	if ( $posted_field->id == $field_id ) {
		$previous_value = FrmProEntriesController::get_field_value_shortcode( array( 'field_id' => $field_id, 'user_id' => 'current' ) );
		$_POST['item_meta'][$posted_field->id] = $previous_value ? (int)$previous_value + 1 : 1;
	}
	return $errors;
}