The District Attorney can dismiss that count and prosecute you on the rest of the indictment. If it's an accusation, the DA can file an amendment. If the defendant was indicted by the Grand Jury and the DA wants to prosecute on all counts, the case will have to go back before the Grand Jury.
Generally speaking, many times an amendment is allowed by the court, especially if it is something small, like a typographical error (a scrivenors' error is what we attorneys call it). If it is a significant change in that count, only that count would need to be re-indicted and the original count in the original indictment dismissed. Your attorney can answer these questions with certainty in your jurisdiction
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Indictments cannot be amended so if one count is not drafted properly then the whole indictment will have to be re-presented to the Grand Jury. Accusations, which are used in misdemeanor and some felony cases, can be amended.
They have the option of (1) dismissing the count and proceeding on the remaining 12; (2) reindicting the entire case; or (3) dismissing the count, reindicting on that count only and filing a motion to join the new indictment with the remaining 12 counts of the old indictment. Normally, they will either dismiss the single count or reindict the entire case.
If you are facing this many counts in an indictment, you need legal representation. One count is not going to make a major difference in your sentence unless it is the most serious charge of the bunch. If you do not have counsel, you need to retain counsel immediately. If you can't afford one, make sure you apply for a court appointed attorney. Whatever you do, don't try to handle this yourself.