According to my understanding, the IWM has
two enable pins (/ENBL1 and /ENBL2), which allow it to natively control two different floppies, so the simple answer is two. But beyond that, it’s possible for software and hardware on either side of it to cooperate in a way that enables additional drives. I’m not sure if the total count of supported drives is well-defined.
The IIgs supports six drives, but
only two of each, and only in specific orderings. The newer, smarter drives (like 3.5” floppy drives) must be earlier in the chain than older drives (like 5.25” floppy drives). I think that the way this works, 3.5” drives understand and ignore 5.25” commands, and pass them downstream, or consume 3.5” commands and suppress them downstream. That way, a 3.5” and a 5.25” drive can share the /ENBL1 signal and both be “floppy 1”.
Apple II daisy-chaining involves putting both /ENBL1 and /ENBL2 on the DB-19 connector; drives connect the upstream /ENBL2 pin to the downstream /ENBL1 pin (and maybe the reverse?). Macs assign /ENBL1 to the internal connector and /ENBL2 to the external connector (
Mac Plus schematic). However, there’s an entirely different way that Macs can daisy chain HD20 drives using the Phase3 pin.
I’m not sure how exactly the SE/LC could support three floppies. It’s possible they have an IWM variant with three enable pins. It’s also possible the enable pins come from somewhere else—I don’t know if floppies actually need their /ENBL signal to come from the IWM, or if it would be possible to enable them some other way.