Every person arraigned for any crime punishable with death, or imprisonment for life, shall be admitted on his or her trial to a peremptory challenge of twelve jurors, and no more; every person arraigned for any offense that may be punishable by imprisonment for a term exceeding eighteen months and less than life, shall be admitted to a peremptory challenge of six jurors; and in all other criminal trials, the defendant shall be allowed a peremptory challenge of three jurors. The attorney prosecuting on behalf of the state shall be admitted to a peremptory challenge of twelve jurors in all cases when the offense is punishable with death or imprisonment for life, six jurors when the offense is punishable by imprisonment for a term exceeding eighteen months and less than life, and three jurors in all other cases; PROVIDED, that in all cases where alternate jurors are called, as provided in section 29-2004, then in that case both the defendant and the attorney prosecuting for the state shall each be allowed one added peremptory challenge to each alternate juror.