Specifically what kind of drives? That might make a difference because of power requirements and specific ways to connect them. Some drives don't require a lot of bus power and you can operate them off of unpowered hubs. If it's going to take a long time you probably want to be connected to power, which will take up one of your USB-C ports. If it's not then you can use some sort of adapter. If it's a common USB 3.0 drive with a detachable USB-A to micro-B cable, then you could use a passive USB-C to USB-A adapter, or you could get USB-C to micro-B cables.
There are many different styles of passive adapters. Some come with little cables while others are fixed and don't bend. Here's a two-pack of ones with little cables:
Amazon.com: AUKEY USB C to USB 3.0 Adapter, [2 Pack, 0.5ft] USB Type C OTG Cable Adapter for MacBook Pro, Samsung Note8 …
Here's the kind of cable that you might need if it's a common external drive or enclosure with a micro-B port at the drive.
Amazon.com: Cable Matters USB 3.1 Type C (USB-C) to Micro B (Micro USB) Cable in Black 3.3 Feet: Cell Phones & Accessori…
You might also use a USB-C to USB-A hub if your external drives don't use much power. For example I've got a Seagate Backup Plus Slim 1TB drive that requires less than 200 mA. I could easily use that with a single hub. I also wouldn't worry about the speed as it should be able to handle the traffic just fine, and you're basically going to be limited by the bulk data transfer speeds of the hard drive as well as seek time delays.
You should be able to do a simple drag and drop if you're not doing a complete copy.