There is no fixed quantity rule for minimum order placement.

The Brand or Project Manager normally determines this based on the project plan, budget and lead time.

Be practical. Samples are made when needed and normally this is because there is a production order in place to back up the request.

When requesting services, consider to also provide a summary of your needs. Opening inventory to support advance sales may suggest that placing a small batch production is a good way to try a styles sales performance when exhibiting direct to consumer or offering items online. take samples from the small lot to pioneer larger prospects.

This way if a buyer requests a sample you have some stock on hand or otherwise plan far in advance when Sampling for buyers and only the items that attract significant interest from qualified buyers or distributors become a production run.

Your Minimum Order really depends on method of sale, distribution and overall business model. If selling direct to consumer at events then make enough but not so many that all your cash is tied up and you have no ability to transact other needs.

Short Answer: Order what you need to be sustainable. More important that you plan in advance and can control the raw material supply. Fabric is key. The devil is in the details.