What is plural and what is possessive for Witness?

I know I'm supposed to know this stuff but it always gets confusing.

How do you write, "The witness' testimony" if you're talking about one witness and again if you're talking about more than one witness.

If anyone could give me the same example for "Nurses assistants," that would be greatly appreciated.

Thank you.

One witness:  witness' testimony.

Several witnesses:  witnesses' testimony.

One witness:  The witness' testimony.

More than one witness:  The witnesses' testimony.

They are both pronounced the same so it may be difficult to differentiate.

Thanks for the help.

Hello, Amandacat.

I'm sorry to report thta you have been given incorrect information.

One witness: "Witness's testimony."

More than one witness: "Witnesses' testimony."

Never write witness' testimony. Ever.

I stand corrected.  Thank you for clarification.