You can create any number of custom fields for your portfolios.

To create a custom field for portfolios:

1. Log in as Administrator (or a user who has the ‘Manage Customers’ permission). If you are already logged in, skip this step.
2. Select Custom fields from User menu | Settings.
3. Select Portfolio under 'Custom fields for'.
4. Fill out the form.
5. Click 'Add'.
6. Repeat steps 4 to 5 to add more custom fields.

The rich text area-type field is not available for portfolios.

You may want to be able to assign a portfolio its internal number in your accountancy system.
Solution: Create a custom field for portfolios named 'Internal number' with the type 'Number' or 'Text'.