Why my iMac is not allowing me to sign in with a new Apple ID?
That Mac is likely now locked out due to automated processing to detect fraudulent account creation.

You'll want to use it with one of the 10 AppleID you made initially on it. I presume that after several weeks or months you might be able to make another AppleID - but you likely got flagged as abusing the AppleID system and would need to contact Apple and have your IP address white listed so you can create a large number of new Apple ID.

They do this for schools and businesses that need to make a lot of Apple ID - reach out to your Apple Sales contact or get together a list of the serial number of the Mac you have so they can see why you need so many new Apple ID at your site.

Otherwise, I would stick to making one test account and one production account and working with Apple to clear / reuse your test account for testing purposes. There are formal testing programs for education, business and if you are a non-profit - you might easily fall under either program to set up some test accounts for testing that don’t belong one-to-one to actual people using different devices.