Ansem is a Premium Box Tsum. He is a Limited Event Tsum and is not available from purchasing Premium Boxes outside of the Event. He was first introduced during the Japan Event Kingdom Hearts (February 2019) and the International Event Kingdom Hearts (May 2019).
Ansem will clear 4 vertical lines of Tsums, then a larger vertical line. His base number required is 30 and his base radius is SS. Higher Skill levels reduce the number required.