If you're creating a document in Google Drive (Google Docs in its previous incarnation), there are only 19 fonts available as default choices. However, after a recent update there more than 450 available if you choose to add them.
Within a document, click on the font list in the toolbar and then choose 'Add fonts'. The pop-up dialogue lets you choose additional fonts to use. As well as a list of everything, you can see just display, handwriting, serif or sans serif fonts, and sort the list by date added or popularity. There's no option to automatically add every single font, but that's a good thing: scrolling through a list of 450+ fonts takes a long time.