That's actually kinda hard, no?

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.

The solution is to hide deep into the Terms of Service "Duplicate numbers may occur. You reserve the right to be sued if you complain."

Maybe just show the last purchased number? Then it’s a race to get your order in if you want the next sequential number.

Exactly why we kept it simple. You find out your number when it ships.

IIRC many websites (e.g. for buying concert tickets) have a lock mechanism where you have X amount of time to make your purchase during which time only a limited number of people can be in the checkout process.