What you could do is add a ‘Control’ table. This is just a table with a single record that is linked to all other records. This will allow you to count the number of records.
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Add a filter view:
And then make a form with the control as a ‘required field’:
If you’re after the 100th customer you won’t be able to submit the form:
This is hacking quite a few Airtable features so it might not be what you were hoping for. Feel free to copy out the base. And let me know if you have any questions.