There’s no minimum purchase amount. You can order a single item or as many as you like.
Order value does affect shipping, though. Inside the EU, orders over about EUR 96 ship free to a pickup point, such as a parcel locker; delivery to a home address is charged. Outside the EU, orders over USD 96 can qualify for free shipping. Otherwise there is a shipping charge that depends on your country and the delivery option you choose, so it is worked out at checkout before you pay.