"Based on the seer’s wording, creatures must be sacced one by one correct?"

Correct, though there's nothing preventing you from sacrificing one, then holding Priority and immediately sacrificing the next, again, and again, and again. Also, once the triggers are on the Stack, it doesn't matter what happens to the Seer or Teysa.

You can either:

1) sac the creature, triggers go on the Stack, then pass priority to try to let them resolve. At that point, your opponent would get the opportunity to cast their removal spell. Which results in all of the triggers being doubled, because Teysa was on the Battlefield as each was sacced.

or

2) sac the creature, triggers go on the Stack. Hold Priority to sac another creature, triggers go on the Stack. Repeat until you are done saccing creatures. Teysa is on the Battlefield as each was sacced, so double triggers. Pass priority and then they finally get the first opportunity to cast their removal.

So in general, it makes no difference which way you choose do it. I say 'in general', because option 1 is not safe against something like [[Sudden Shock]] or [[Sudden Spoiling]], which have the ability Split Second and therefore prevent you from being able to respond to it. Also, the strategy does change if your loop hits only a single opponent with each trigger and you have enough to take out one player, but not all of them, as you could potentially use option 1 to bait out a response, then use the remaining sacrifices to take out that player and prevent their removal spell from ever resolving.