Pre-qualify a number entry

Okay, if we disregard the potential for race conditions resulting in multiple users claiming the same slot, then the way I would approach it is with a Custom Form. Capture the slot number in a User Specific column, and use that to build a relation to the table where all the taken slots are stored. If the relation is not empty, then the slot is already taken.

For a more robust option that eliminates the possibility of race conditions, you might want to use something like Upstash or Make to manage the allocation of slots. Have a read of the below, and the reply from David:

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