The disclosure of witnesses should be governed by the scheduling order. Most court's frown upon naming previously undisclosed witnesses shortly before trial because that defeats the purpose of discovery, which is to avid trial by ambush.
DISCLAIMER: David J. McCormick is licensed to practice law in the State of Wisconsin and this answer is being provided for informational purposes only because the laws of your jurisdiction may differ. This answer based on general legal principles and is not intended for the purpose of providing specific legal advice or opinions. Under no circumstances does this answer constitute the establishment of an attorney-client relationship.
Because you do not disclose what kind of "hearing" you are talking about it is very hard to give assistance. If this is a civil trial in superior court, there are rules about exchanging witness lists. Once the deadline for exchanging and filing witness list has passed, you can generally not add new witnesses unless you can show good cause, or if the witness is being used solely for impeachment purposes. The best way to protect yourself against a party bringing in a "significant number of new persons as witnesses" is to engage in discovery that requires the other side to disclose to you all persons within their knowledge that may have facts related to the various issues in the case. A failure to disclose a witness in discovery can sometimes be used to prevent surprise witnesses from testifying in trial.
You have probably heard this before, but it is important enough to say again: trial is a hugely complicated endeavor. Many attorneys do not even want to go to trial because of its complexity. You are going to be grossly outmatched if pitted against an experienced trial attorney. It will be like an average person with a high school education going up against a PHD in classical studies on Jeopardy. If you have the ability to do so, hire an attorney to try the case for you, or at least shadow you.
Good luck to you.
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Your question demonstrates why persons need to hire attorneys. If you do not have one, get one!
The above is not intended as legal advice. The response does not constitute the creation of an attorney client relationship as this forum does not provide for a confidential communication.