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That depends on your operating system and hardware.

There is a limit to how many USB devices can be attached to a USB controller. Any hubs on the controller count as a device, too.

A port has limited power, and USB drives have a tendency to absorb as much as a port can (usually) give. And that power is divided between devices in a non-powered hub.

But bandwidth of the USB will be problematic - a USB2 device only goes USB2 speeds, even when plugged into a USB3 port, so your USB3 port cannot talk at USB3 speeds when USB2 devices are attached and active. I have played with 2 USB2 drives on the same USB controller, and that works (optical read speeds leave big enough gaps to interleave at least 2 drives worth of data). But I have also seen significant slowdowns when the network controller is also a USB device.

As far as MakeMKV is concerned, though, it's up against operating system limits for block devices if you "go wild" with lots of hardware.

Since I like to preset the file names when I start a rip, I found the practical limitation on my time was 5 drives, because the first would generally finish ripping before I could finish setting up the 6th disk for ripping. These weren't USB drives when I tested this, but SATA drives split between multiple computers.

Your connection to the storage device can also limit what you can do.

If you use a magnetic hard drive then don't forget that the read/write head will start to move around reducing the writing speed dramatically. When I copy a big file from A to B then my hard drive is so bussy that it isn't even fast enough anymore to store the data comming from 1 DVD drive (about 12 MB/s).