Too Many Fonts Is No Good
The above photo is a representation of the same type of chaos we experience when too many fonts are used in a single space. When we think of design, we think of words like "cohesive" or "balanced". Everything fits in the right place, and aesthetically speaking, we get a sense of warmth and comfort. When we use too many fonts, all of these feelings go right out the window.
Too many fonts is like having too many cooks in the kitchen. Everyone fighting for attention and a place to fit in. The words that come to mind are "chaos" or "unsettling". By keeping out fonts down to two or maximum, three, we can help users better navigate our websites and find exactly what they are looking for. Let's take an example:
We can see that by having multiple fonts all interacting in the same sentence, it's really hard to find a focal point and keep our attention. If we can have two main fonts that we use for our headers and body text, and maybe one extra to help place accents here and there, we can achieve a better overall experience for our customers. It also helps us strengthen our overall brand and builds trust with our audience.
I made this flyer for a local brewery here in Chicago, notice I used one font for the introduction - and only one other font for the rest of the flyer. It takes some of the weight off of our shoulders as well, because we don't need to make twenty choices as to what font we want to use, we pick two and stay the course!

Keep It To Two, Three Max
By keeping it to two fonts, three maximum we can hold our readers attention longer and get our point across. People don't have the attention spans they once did and if something isn't displayed properly it won't be more than a second before that window is closed or attention is pushed elsewhere. Simplicity is bliss! Keep it simple and you will find more success!