Adding an 'auto_increment' field

There's now a module to do this, Serial Field, I recommend you to use that one instead of this option

This snippet will create an auto increment field. You could use this if you have a content type where you need to register a transaction number and you don't want to let the users add the transaction number because you might lose the sequence.

if (!empty($node_field[0]['value'])) {  // the node is not new
  return $node_field[0]['value'];
}
else {  // the node is new
  $count = db_result(db_query("SELECT COUNT(*) from {node} WHERE type = 'transaction'"));
  $node_field[0]['value'] = $count;
}

In this case the Content Type is 'Transaction' so you'll have to change that to the content type you want to count...