When you're doing a backup on a tape, there should be a progress indicator that tells you how much space you have or how long it will take until the tape is full. In most of the cases, the backup is done on multiple tapes as a measure to prevent data loss, but if the tape will reach its end, then an error will be shown on screen or in the software that you're using to do the operation. Multi-session is also possible, but this involves in having additional knowledge.
Secondly, everything relies on the server configuration and the way it's set to make those backups. The configuration and architecture are key factors when TAPE backups are created since no system and configuration is the same.
