Only amateurs call typefaces "fonts", you know? "Proper" design professionals call them "typefaces" Fonts are the files on the device that the software uses to render the typeface. Fonts are the paint on the palette, while the typeface is the masterpiece on the canvas.

Regardless, too often designers add too many typefaces to their products. You should aim to use two typefaces maximum: one for headings and titles, and another for body copy that is intended to be read.

Use weights and italics within that font family for emphasis—rather than switching to another family. Typically, this means using your corporate brand font as the heading, while leaving the controls, dialogs and in-app copy (which need to be clearly legible) in a more proven, readable typeface.

Using too many typefaces creates too much visual "noise" and increases the effort that the user has to put into understanding the view in front of them. What's more, many custom-designed brand typefaces are often made with punchy visual impact in mind, not readability.