Answer
20,480,000 characters
Work Step by Step
Number of tracks in one surface = 500
=> Number of tracks in two surface = 1000
Number of sectors in one track = 20
=> Number of sectors in 1000 tracks = 20*1000 = 20000
Number of characters per sector = 1024
=> Number of characters in 20000 sectors = 1024*20000 = 20480000