each time the form is supmited, you would need to use an insert recrd behavior to store the form contents to a database table.
you could then create a recordset that will return all records from the table:
SELECT * FROM tableName
the number of rows returned by the recordset will be stored in a variable created named:
$totalRows_<recordsetName>
where <recordsetName> is the name of the recordset.
you could use an if statement around the form:
<?php if($totalRows_<recordsetName> < 100) { ?>
<form...
form code
</form>
<?php } ?>