Purchase minimums can be set storewide or by customer type for a more customized experience.
To set storewide minimums that apply to all customers in both the B2B and B2C store, navigate to Settings / Edit Site Options / MInimum Order Quantity (on the right side of the page). This allows you to set minimum dollar amounts or minimum total item quantities for every purchase in the stores. Be sure to read the ? tooltip at this location for all the fine details. When this feature is activated, no customer can check out until the minimum has been met or exceeded.*
MINIMUMS BY CUSTOMER TYPE
For merchants supporting different experiences for different types of B2B and/or B2C buyers through their online stores, a more granular approach likely will make more sense than storewide minimums. To set minimums that apply only to specific customer types (regardless of whether they shop in your B2B or B2C store), navigate to Customers / Types / Define Minimum Order Price/Quantity by Customer Type. On the following screen, you will see a list of Customer Types, and a field next to each where you may enter minimum order price or minimum order quantity values. Like the storewide minimums if customer-type minimums are set, customers may not complete the checkout process until they have satisfied the minimum requirements for their customer type.*
*Take care not to confuse this functionality with the Minimum Order Quantity on the Product and SKU levels. The latter applies to an individual product or SKU only, while this functionality applies to all products in an order.