External hard drives are generally either USB or firewire. Firewire devices often have an inbound port as well as an outbound you can just keep piggybacking them on each other, and I think some USB drives can do this as well.
On a laptop you'd just need an available USB/Firewire port, or a USB or Firewire PC card and available ports. On a PC you'd need available ports or to install a PCI card like
this. (note: I'm not endorsing that product, it was just the first relevant example I saw when I googled for a firewire card)
I recommend Firewire devices if speed or latency is an issue, but if you just want the best capacity per dollar, the USB spec is older, more likely to exist as an available port on your computer without needing an interface device, and generally cheaper.
I'm not really sure why you are concerned with the max number though, as you can just swap them out if you run out of ports or whatever.
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