I'm not sure if this is the question exactly, but itt was a 200gb hdd with 100gb backups I think, with Monday-Thursday being incremental 100gb. Each tape is 50gb.
So do the maths, 2x 50 tapes, x 4 days = 8 tapes there, and then the full backup is 4 tapes for Friday. All up 12 tapes.
I hope...