Why is the Basmala (Bismillaah Ar-Rahmaan Ar-Raheem) counted as the first verse only at the beginning of Surat Al-Fatiha, and not in other surahs?

The scholars have agreed on writing the Basmalah in the Mushaf at the beginning of every Surah, except for Surah Bara'ah. However, they differed on whether it is an Ayah of every Surah, or only of Al-Fatihah, or merely for separating Surahs, or if it is not part of the Quran except in the Ayah of An-Naml. Thus, whoever states that it is not an Ayah at the beginning of the Surahs does not commit disbelief, because it is a matter of scholarly disagreement. But whoever denies that it is part of the Quran commits disbelief due to its presence in Surah An-Naml.

The Medinan count does not consider the Basmalah an Ayah of Al-Fatihah or any other Surah, whereas the Kufan count considers it an Ayah of Al-Fatihah only. This is what we find in the Mushafs of the Hafs recitation. As for the Mushafs of the Qaloon and Warsh recitations, they do not count the Basmalah as an Ayah.

Despite the agreement of everyone on writing it at the beginning of every Surah except Bara'ah, and on the fact that Al-Fatihah has seven Ayat, whoever counts the Basmalah as an Ayah does not count "صِرَاطَ الَّذِينَ أَنْعَمْتَ عَلَيْهِمْ" (the path of those upon whom You have bestowed favor) as a separate Ayah. And whoever does not count it as an Ayah counts "صِرَاطَ الَّذِينَ أَنْعَمْتَ عَلَيْهِمْ" as an Ayah, followed by "غَيْرِ الْمَغْضُوبِ" (not of those who have evoked Your wrath).

In conclusion, counting the Basmalah as an Ayah of Al-Fatihah alone, without other Surahs, relies on the Kufan count. The number of Ayat is a matter of divine revelation (tawqifi) and depends on the narration from the Prophet, peace and blessings be upon him.