If you press Enter after entering an e-mail address, a vertical scrollbar appears and you automatically get a new recipient line.

Alternatively, you can use the Contacts sidebar by pressing F9 and select recipients there and click on the Add to To: button.

How do you address your messages? Type in the addresses? Pick them from the list at the left?

I use auto complete. I type in a few letters and the system suggests matches from the address book. I pick which one I want then hit return to add that address and the cursor moves to the next box.

If you use the Contacts Sidebar, just keep using the "Add to..." buttons

You should see a scroll bar at the right of the addresses indicating when there are more addresses than can be fitted in.

You can drag the separator between the addressing space and the message space, making room to display more addresses.