Currently using three of the four USB 3·0 ports on the iMac plus 2 of the three USB 2·0 ports on the Logitech AudioHub (powered speaker/USB hub). One of the two Thunderbolt 3 (USB-C) ports is free, the other has a Thunderbolt 1-to-USB 3·0 & Gigabit Ethernet adaptor attached in turn to a T/Bolt3-to-T/Bolt 2 adaptor. Done this way I can connect USB thumb drives to USB 3·0 without pivoting the iMac to access the ports at the back.
In another week or so I'm receiving a 4-port USB 3·0 front-facing clamp-on hub.