minimum order of 10 DIFFERENT items

Is there a way of making it so that a customer has to buy a minimum of ten items (but not necessarily of the same item) from a selected range of products. Plus I want it so that there is a discount triggered for buying 20 items.
like

10 items @4.95 per item = 49.50 in total
20 items @4.45 per item = 89.00 in total

minimum order of 10 items

any help would be appriciated