What adapter do I need for USB keyboards on iMac?

The 2023 iMac's ports are Thunderbolt/USB-C.

What you need to plug the keyboard and other USB-A devices into the iMac, are USB-C to USB-A adapters.

for a few examples:

USB-C to USB Adapter - Apple

https://www.amazon.com/dp/B07PDLQ555/ref=sspa_dk_detail_0?

https://www.amazon.com/dp/B08Z768LCW/ref=sspa_dk_detail_1?

https://www.target.com/p/philips-usb-c-to-3-0-usb-a-female-adapter/-/A-79804275

I bought these 3 from Amazon and they work as advertised: USB C to USB Adapter, Type C Thunderbolt 4 OTG Converter, USB C Male to USB 3.0 Female Adapter (3-Pack) for Apple MacBook Pro, Mac Book, iPad, Samsung Galaxy S20-S23 and More (Black/Grey/Silver). And the price was right.

I have never heard of a Thunderbolt keyboard, I suspect you are mistaking USB C with Thunderbolt. They both use the same connection type. You may need something like USB to USB C adapter. At any rate, please look on Amazon you will be able to easily find an adapter.

+1👍

You're welcome.