Scepter says "create
a runechant token for each damage dealt this way"
The key to look for is how many damage events there are, not how much damage is being dealt.
If a card were to add another damage event to scepter it would be a different story.
To better understand it you could replace the words in the sentence with the values being applied.
So let's say Amp 1 is in effect and scepter is activated. No damage gets prevented.
"Deal 1 arcane damage to any opposing target.
Create a Runechant token for each damage dealt this way."
Now let's apply amp replacement effect.
"Deal 1+1=2 arcane damage to any opposing target.
Create 2 Runechant tokens."
So we see here they're is a single event creating 2 runechant tokens.
Now if they was a different effect that wasn't a replacement effect adding more arcane damage to the scepter then it would be multiple instances and would result in multiple creation effects.
As a non legal example.
Sting of sorcery gives all your attacks action cards "when this attacks deal 1 arcane damage"
Let's pretend Scepter was an attack action card.
It would then be
"Deal 1 arcane damage
Deal 1 arcane damage
Create a runechant for each damage dealt this way"
Now in this case because it's not a single instance of 2 arcane damage but instead 1 arcane damage 2 times each 1 damage would create a runechant token. And in the case of Florian and mordred tide, each creation event would generate an additional token.