24 quarts would be about 23 kilos. Xantham gum is generally used for thickening in an amount of .15% to .5% by weight. So the expected range would be 34 to 115 grams. Start small, thickening with xantham gum can get snotty. Work up slowly, mixing thoroughly, until you reach the thickening you want. You may find that a combination of cornstarch and xanthan gum is ideal.