Is a notary public the only signature, other that the agent, that needs to sign a POA?

Two witnesses here too, and there are restrictions on who can be a witness (must be impartial).

In my state, it's 2 witnesses and acceptance by the person agreeing to act.

TKRenick, my Elder Law Attorney had signed my Power of Attorney. The Attorney had prepared the legal document, it was 17 pages long and covered a lot of important items.

Some States require the person who was assigned to be the Power of Attorney to also sign the document, and that signature also needs to be notarized.

You may also need one or two witnesses, depending on the laws in your state regarding these documents.