What is the ruling for one who performs only one prostration of forgetfulness (Sujud Al-Sahw), and is it sufficient to rely on the opinion of Imam Al-Subki regarding this matter?

The prostration of forgetfulness (sujud al-sahw) consists of two prostrations, not one. No scholar has ever said it is a single prostration. Shafi'i jurists have explicitly stated that sujud al-sahw consists of two prostrations, and if someone performs only one prostration, intending it to be a single one, their prayer becomes void. They even stated that if the imam performs only one prostration, the follower should perform two.