We currently support Google Fonts. You can see all available fonts at fonts.google.com.
While this means that some more traditional fonts, like Helvetica, aren't available, there is usually an equivalent available in Google Fonts.
If you’re someone who needs to match a font, Google something like “Helvetica equivalent on Google Fonts", and you'll find some suggestions.
You can also bring any other font you'd like using Custom CSSon the Agency plan by using the @font-face selector if you have the know-how.