Does the mahr amount need to be announced at the nikah?

No, it does not need to be announced at the nikah in public. Meaning no one other than the bride, the groom, the wali, the guardian, witness number one, witness number two, needs to know what the mahr is.

Because they are witnessing to a contract, they need to know what that is.

But it does not need to be announced.

And in fact, I don't do weddings anymore but when I did the signing was done privately and then the announcement public, when it was announced publicly, I would say that, so and so was offered the hand of so and so in marriage by her father, and he accepted, and she accepted him, and so according to this offer and acceptance, and given in front of these witnesses, and for the mahr that was agreed upon between them, they are married.

That's it. That's all that needs to be done.