It's not common, but we get calls from our vendors saying "okay, you sold too many, you need to end it now otherwise we're going to have fulfillment problems".
All drops have limits. Even manufacturers only have a limited volume of stock available. Most companies like to keep stock low and do manufacturing runs on request so they don't get stuck with any excess items.
We try to keep stock availability as high as possible but vendors only have so much stock available at a given time. Sometimes that's hundreds of units, sometimes that's 10 units. We're working on a better way of clarifying this on our product pages and we appreciate your patience in the meantime.
Thanks for your comment! Let me know if there's anything we can help you with and we'll keep looking for more FiiO X5s in the meantime.