Measure one representative recording
Convert a typical title with the settings you plan to use. Note its duration and output size. Different recording lengths and picture detail can change the result.
Scale the estimate
For example, a hypothetical 90-minute file at 1.8 GB would suggest about 36 GB for twenty similar titles. This is planning arithmetic, not a measured BROOVIS compression result.
Budget for backups and working space
Keep enough free space for temporary work and at least one additional copy. Do not choose a smaller size simply to fill the last available space on a drive.