From what I recall from the time I still used MS-DOS, even if you only have one floppy disk drive, it's accessible as both A: and B: and it prompts you to insert the other floppy disk when you change the drive letter. That is, it's "virtualizing" two floppy disk drives using a single physical one. That explains why the first hard disk drive was always C: even when you only had one floppy disk drive (and of course you had at least one floppy disk drive, how could you use a computer without one?)
> and of course you had at least one floppy disk drive, how could you use a computer without one?
They were relatively rare, but there absolutely were DOS-era machines without a floppy drive – found in schools, corporations, government agencies.
The most common implementation used Novell NetWare's RPL protocol to boot using an Ethernet card with an installed RPL BOOT ROM. The machine didn't need any disk drives (hard disk or floppy) at all, it loaded MS-DOS off a NetWare server, which was also used for all files or data.
You could also install a hard disk for local storage, without installing a floppy drive, but the more common configuration was no disks at all.
Also, the original IBM PC supported external floppy drives; as the 80s went on, external floppy connectors became increasingly rare, but if you specifically wanted an ISA FDC card with an external connector, they were still making new ones into the 1990s. (These didn't use anything like USB; they took the signals of the internal floppy ribbon cable, and surfaced them on a port on the rear of the card.) So, you could have a machine with a hard disk but without a floppy most of the time, and you'd plug in an external drive when you needed it, and you could keep it locked in a cupboard out of the hands of the plebian users the rest of the time. I doubt many people did this – diskless workstations were a lot less work – but probably somebody did at some point.
> of course you had at least one floppy disk drive, how could you use a computer without one?
IBM PCs, even into the PS/2 days, would boot into Cassette BASIC if you didn't have a floppy drive or boot disk inserted: https://en.wikipedia.org/wiki/IBM_BASIC
My high school had the PS/2 Model 25 SX, introduced in 1992, without hard drives. All of the software ran off of a Novell NetWare file server, but required a bootable floppy disk. If you removed the floppy disk, they would boot into the built-in BASIC from the machine's ROM.
Which is very convenient (and necessary for backwards compatibility) because programs could be (and were) written to assume they're in B: (and the DOS disk is in A:), or that they're in A: and the user's data is in B:, or a two-disk program is in both. All of that just worked (tm) even with a single drive, as long as the program didn't try to interleave accesses to both drives (I think it would still work because DOS was single-process, single-thread, it would just annoy the user!)