Max no of HDDs with Windows 7

[#1]

Is there a maximum number of hard drives you can, sensibly, attach to a Windows 7 PC?
I have used up all the internal bays (5) and have many external drives of various sizes for varying jobs including local backups. Each external drive has its own power supply.
Rather than swap then about I was wondering whether any issues could be caused by using one or two (4-port) USB hubs as, with all other peripherals connected, I have run out of spare USB ports on the case.
I have a 750 Watt power supply with plenty of fans so the internal drives are fine. I currently have 12 (5 int+7 ext) but would I possible meet some issues if I add any more?

[#2]

I'm guessing you might run out of drive letters, unless you run as a server...

[#3]

I wasn't quite thinking of that many 

[#4]

The short answer is that there is no effective physical limit.
The long answer is that you can mount hard drives as drives, or into folders, or you can stripe multiple physical drives together into one logical drive and mount that.  Or you could install a raid card which will do the same thing but present itself as a single physical drive to the OS.
You'll also be "somewhat limited" by the USB bus limit of 255 devices.
For what you're asking though, no - there is no real limit that you would reasonably reach
B.

[#5]

Thanks Barry. You just confirmed what I thought