Complex sentence: The number of music and dance performers in the imperial court reached tens of thousand, most of whom specialized in "yanyue", and all under the administration of the Drums and Pipes Bureau and an umbrella organization called the Taichang Temple (太常寺).
Split into simpler sentences: The number of music and dance performers in the imperial court reached tens of thousand, most specialising in "yanyue". All were under the administration of the Drums and Pipes Bureau and an umbrella organization called the Taichang Temple (太常寺).