My question is regarding the number of compulsory sujoods in the Quran. The sujood symbol comes up 15 times in the Quran but sayed sistanis website only mentions 4, what is the reason for this?

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There are only four compulsory sojoods in Quran.
Those are in the following suras:
Sajda, Fusselat, Najm, and Alaq.