Shiva Purana
व्यासाश्चैवाष्टविंशत्का द्वापरेद्वापरे विभो॥ योगेश्वरावतारश्च प्रारंभे च कलौ कलौ ॥ ५२॥
O great lord, the Vyāsas in every recurring Dvāpara are twenty-eight in number. The Yogeśvara incarnations at the beginning of every recurring Kali age are also so many.
english translation
vyAsAzcaivASTaviMzatkA dvAparedvApare vibho॥ yogezvarAvatArazca prAraMbhe ca kalau kalau ॥ 52॥
hk transliteration by Sanscript