well i would say for 2TB of data you would need about 5 Tapes. if it is more than 2TB it can go to 6 but as already have been told, it depends on the data you backup. in dp go to device and media -> media -> pools. then click on the media -> properties -> usage. there you should see how much data is on it if the tape is full.
is write compression enable in data protector?
device and media -> device -> in the properties of the device go to Drives tab and check if the tape address does end with an C. for example tape0:0:1:0C. If so, compression in DP is enabled.
If you have an HP Library or drive, you can use HP Library and Tape tools to enable/disable compression on the tape itself. you can also do some write testings to see if the compression is enabled.
But if you backup already compressed image files, you can not expect big compression with backup.
regards