A little oopsie here in the manga, I think, but Amakusa here is cutting off his right arm, which has Semi's command seals.
As mentioned, Amakusa's CSs appear to actually be on his left arm, at least, according to the LN illustrations and in one scene that I could find in the Anime. I'm not sure if it is officially stated anywhere in text, though. On the other hand, the Manga has continuously showed the CSs on his right arm, which leads to the situation above.
Just something to note.
“I’m not the sixteenth, Chiron. The sixteenth is Ruler beside you. Strictly speaking, I am the first Servant.”
“Master of Assassin… What have you done to our Masters!?”
In response to Archer of Red’s enraged question, the youth chuckled and held up one arm, rolling back the sleeve covering it. Everyone gasped as they looked at it.
Engraved on it was all the respective Command Spells for Archer of Red, Lancer of Red, Rider of Red, Berserker of Red, Caster of Red and Assassin of Red—eighteen Command Spells in total.
“I peacefully had them turn over their rights as Masters and each of their three Command Spells. Don’t worry, the prana necessary to keep you all in this world is a trivial amount to supply now that I’m connected to the Greater Grail.”
“Master of Assassin… What have you done to our Masters!?”
In response to Archer of Red’s enraged question, the youth chuckled and held up one arm, rolling back the sleeve covering it. Everyone gasped as they looked at it.
Engraved on it was all the respective Command Spells for Archer of Red, Lancer of Red, Rider of Red, Berserker of Red, Caster of Red and Assassin of Red—eighteen Command Spells in total.
“I peacefully had them turn over their rights as Masters and each of their three Command Spells. Don’t worry, the prana necessary to keep you all in this world is a trivial amount to supply now that I’m connected to the Greater Grail.”
At Lancer of Red’s question, Shirou nodded and raised his right arm. The countless Command Spells engraved on it shined with a dull light.
“I’m sorry, but I won’t be able to command you all on the front lines. However, that will make it difficult for me to immediately react if Ruler uses her Command Spells. Therefore, I will use two Command Spells on each of you and raise your resistance to any Command Spells from her.”
Both their Master’s Command Spells and Ruler’s Command Spells had equal power of compulsion. Therefore, by limiting the activation conditions and using two Command Spells to do it, Shirou could repulse Ruler’s Command Spells with almost no time loss.
“I’m sorry, but I won’t be able to command you all on the front lines. However, that will make it difficult for me to immediately react if Ruler uses her Command Spells. Therefore, I will use two Command Spells on each of you and raise your resistance to any Command Spells from her.”
Both their Master’s Command Spells and Ruler’s Command Spells had equal power of compulsion. Therefore, by limiting the activation conditions and using two Command Spells to do it, Shirou could repulse Ruler’s Command Spells with almost no time loss.