If you need to add new fonts you can add them in the RTE Profile in the core
database:
Note that this is the Rich Text Full
profile, by default Sitecore is configured to use the Rich Text Default
profile:
<setting name="HtmlEditor.DefaultProfile" value="/sitecore/system/Settings/Html Editor Profiles/Rich Text Default" />
However, I would recommend that you do not use font styles in RTE, it will make your life more tough in the long term and is not best practice. If the business decides to change the default font then you will have to mnaully go through content and change them.
Instead use CSS styles and set the font to use in that stlye declaration. You can link a CSS Stylesheet to the RTE and add your custom styles in there so users can select them. Either update the default css files or link to a custom stylesheet and update the setting in config:
<setting name="WebStylesheet" value="/default.css" />
The users will be able to select the styles in RTE then: