Why it matters: It's not like a fixed rule: sometimes it’s possible to have more than two. Yet, generally speaking, combining more than two font families is not an easy job. For usability and visual purposes, focusing only on two font families are much better, because it'll simplify your typographic hierarchy and improve website's comprehension.
How to test it: Check your website's design for font families. In case, more than two are used, select the pair that matches well and replace the fonts. Read this guide to font pairing: