 He might have made-up the idea of "nine islands" since "naba" in Bengali also means 'nine'.Hence "Nabadwip" i.e.nine islands, namely Antardwip, Simantadwip, Rudradwip, Madhyadwip, Godrumdwip, Ritudwip, Jahnudwip, Modadrumdwip, and Koladwip.