I have multiple letter codes in box 12 of my w2

Yes, you add each code separately.  On the W-2 entry screen, you can click  +Add another row  under Box 12 to add another entry line.

To edit your  W-2:

• Once in your return, type "W-2" into the search box on blue dashboard
• Click blue  "jump to W-2" link
• On "Here's your W-2 info" screen, click Edit next to the W-2
• Under Box 12, click +Add another row to add additional letter codes and amounts.

I am using a Mac, there is no "+Add another row" option or button.

+1

For MAC, it looks like there can only be four lines, but hover over the box and try scrolling up and down. A fifth line will appear 🙂