Q- heard in a lecture that Allah Ta’ala has created 18,000 different creations. Is this correct and can it be substantiated from any reliable source?
Perhaps you are referring to the following hadith which Imams Ibn Jarir At-Tabari and Ibn Abi Hatim (rahimahumallah) have recorded on the authority of Abul ‘Aliyah (rahimahullah) as his statement (Maqtu’an).
Commenting on the first verse of Surah Fatihah, specifically the words رب العالمين, Abul ‘Aliyah (rahimahullah) has mentioned: “Humans are one ‘Aalam (universe) and Jinn are one ‘Aalam. And besides that, there are eighteen thousand ‘Aalams or fourteen thousand ‘Aalams – he was unsure – of Angels on the Earth. The Earth has four corners. In each corner, there are three thousand five hundred ‘Aalams. He [i.e. Allah Ta’ala] has created them for his worship.(Tafsir At-Tabari and Tafsir Ibn Abi Hatim, Surah Fatihah, Verse: 1; Also see: Mufradatul Quran, letters: علم)
After mentioning this narration, Imam Ibn Kathir (rahimahullah) has pointed out that this narration seems strange (و هذا كلام غريب يحتاج مثله إلى دليل صحيح). One should therefore refrain from quoting it until further substantial supports are found. (Tafsir Ibn Kathir, Surah Fatihah, Verse: 1)
And Allah Ta’ala knows best.