If your ODBC driver supports it, you can do:
$numrows = odbc_num_rows($rsD1);
However, many ODBC drivers do not return information for how many rows were in the recordset. Thus, although PHP supports it, many ODBC drivers will return -1;
I'm not sure what drivers do and don't support this. I definitely know that the Microsoft JET driver does not.
As an alternate, you can do an SQL COUNT() on your data.
$strSQL = "SELECT COUNT(requestNumber) as theCount FROM [post_received] WHERE requestNumber = '$D1'";
$rsCount = odbc_exec($conn, $strSQL);
$rsD1Count = (int)odbc_result($rsCount, 'theCount');