For an AS/400, that is an old old machine.
The operating system is called OS/400. The latest version that runs on that machine was V3R1, which was released around 1994 or 1995. Version V2R3 would be a good version to use as well.
I'm not sure how many tapes it takes - dependening on the tape drive, it might be spread over a handful of tapes. Besides the operating system there were licensed program products (compilers, etc.) that would have been on tape as well. Later AS/400s used CD-ROM.
These machines used a CISC processor known as 'IMPI'. Most of the design of the machine (right down the processor) comes from Rochester MN where the machine is designed and built. (There are two other manufacturing facilities in the world for other markets.)
Did you get an operators console for it? It would be a terminal that plugs in using a think 'twin-ax' cable. Twin-ax is a coaxial cable with two conductors. The monitor speaks a protocol to the machine known as 5250. It's similar to the 3270 terminals, but it is AS/400 specific. Oh, and yet another quick - AS/400s use EBCDIC, not ASCII.
So what are you going to do with this thing? Back when it was sold in the late 80s it was a powerful business machine with a great database, COBOL and RPG languages. In 2005 it's too big to be a boat anchor. Unless you have a lot of patience, it's going to take a lot to get it running. You may need to find a suitable console, which isn't too hard - look for AS/400 terminals in eBay. The operating system and software are going to be harder to find.