"Creatures tapped this way" means creatures that became tapped as a result of Suppression Ray's effect, not creatures that were already tapped before Suppression Ray began to resolve. For example, if a player controls two tapped creatures and one untapped creature and you cast Suppression Ray targeting that player, you'll only be able to choose the creature that was untapped to get a stun counter (as long as you paid at least one {E}).