Say the server has a counter. When you load the page, it's at 57, so it displays that you would be ordering #57. While you're looking at this, someone else loads the page - what number do you show them? If you show 57, then whoever orders first gets it and the other person gets a message "Sorry, not available. Want 58 instead?" but the same thing could then happen to them with #58, too – "Sorry, not available. Want 59 instead?"

So maybe instead you show the 2nd person counter+1, i.e. 58. And you show the 3rd person counter+2, i.e. 59. But what if #59 purchases but 57 and 58 don't? What do you show the NEXT person, 57 or 60?

I'm not saying it's intractable but it merits thought.

[1] https://en.wikipedia.org/wiki/Pet_Rock

“All park resources are protected so that all visitors may enjoy them. It is against the law to remove any of the natural (petrified wood, other rocks, plants, animals) or cultural resources (pottery pieces, arrowheads, Route 66 debris), including picking flowers.”

https://www.nps.gov/pefo/faqs.htm

https://aws.amazon.com/bedrock/