Removing choice is fine, but people will want to know what number they'll get if they order right now. Even if you can't or won't show the next rock, show the next number.
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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.
Sequential assignment follows strict order of payment completion only. No race conditions, no held inventory, no time windows. You either complete the transaction and receive the next sequential number, or you don't.
The constraint is designed to eliminate the entire apparatus of purchase optimization, including queue management systems.