Some larger archives have by now exceeded 1 million tapes. NASA for example, has so much telemetry to store that hard disk arrays are used mostly as a staging.
Years ago NASA was unable to find tapes with Apollo photographs. Eventually they failed to find them and its is now believed that the original tapes were overwritten.
1/2″ tape can last for decades in a climate controlled warehouse. NASA has taken advantage of the capability.
The old IBM 3850 tape system was the first to offer a robotic system to handle tapes. Several models of the 3851 were available. The smallest A1 holding 706 cartridges storing 35.3GB, while the largest A4 held 4,720 cartridges storing 236GB in a 20-foot (6.1 m) long unit. All of the units were also available in the “B models” which added a second controller for on-line backups, as well as offline storage. A second series was released on March 6, 1980, doubling maximum capacity to 472GB. The entire series was discontinued on August 5, 1986.
Over the last decade LTO has standardized the industry around one standard and everyone shares patents fairly. The latest standard is LTO-9 which is 18TB per tape native. One LTO-9 tape can store the data of 4720 cartridges from the old IBM 3850 with room to spare.
The problem with some archives in government and banks etc is that legacy tapes should be migrated to LTO which is more robust and capacious. LTO is a solution for archives but some effort towards modernization is need.
The old IBM 2401 tape drives held 170 MB which fits a modern USB stick easily. The advances have been spectacular.
Migrating from 2401 tapes was done when the 3580 tapes came to market with their easier to use cartridge format.
QIC tapes came to market in the early 1970s but mainframes did not adopt the technology. Mainframes instead focused in expanding hard disk capacity. IBM has sold tape systems along side LTO into recent years. QIC tape have dual reels which does complicate the manufacture.
A wide range of proprietary systems from a range of backers all came up short against the power of IBM and the LTO consortium.
LTO tape are single reel with a takeup reel in the drive itself. This makes the system reliable and affordable. Handling 1 million misc tapes in an archive can be a challenge in its own right as the data needs to be organized. Large scale hard disk servers can hold lots of tape data and the directory trees can be matched with LTO so that new archives maintain the older archival intent. Modern file systems are capable of very deep directory trees to make archive management easier.
Copying one robotic storage system to a different one can be done. The performance of one tape is not that fast but multiple drives are able to scale the performance significantly. Modern tape appliances are fairly robust with LTO media as the tapes can be changed for new tapes and drives can be swapped out.