These tires are sold individually. we do sell wheel/tire packages, that include 2 tires, 2 rims mounted. but the product link for this question is just the tires only.
here's the link to our wheel/tire combo deals.
Going from stock 18's to 21's is a big change especially in the sand, if you like the tires try re-gearing it first, going down 1 in the front could help a little and its fairly inexpensive. if not see if a friend has some other sizes to try before you buy.