The new iMac will have four USB 3·0 ports & two Thunderbolt 3/USB-C ports. I plan to connect three external HDs, (clone, Time Machine & general storage) plus a USB scanner, USB laser printer (although that can use Ethernet) and a USB external 2.1 channel sound card/speaker system. Only one HD will use the USB-C/Thunderbolt 3 connection via Thunderbolt 2 & FireWire 800 adaptors.
I've bought a clamp-on unpowered 4-port USB 3·0 hub that uses one rear USB port to direct four USB 3·0 ports to the front of the Mac & the Logitech 2.1 channel speaker incorporates a powered USB 2·0 hub, so I'll have no trouble with needing additional USB ports.