I seem to remember coming across a UX rule several years ago referring to the max number of font styles that should be used on a webpage. Can anyone here who is more involved with UX provide some insight on this? Is the general rule max of 3 where a style is defined as any variation/combination of font attributes such as size (12, 14, 16, etc), face (arial, etc), weight, etc?
The general 'rule of thumb' with most design considerations is that using a smaller number of font styles generally leads to a cleaner design that is also easier to maintain and apply consistently. But knowing the rule allows you to work out when it is appropriate to introduce additional font styles/combinations.
Just to provide a little bit more of an explanation, while this is primarily a visual/graphic design question, the impact of the choices and decisions made on the font style does have a direct impact on the user experience.
Some of these experiences are subjective, for example, there are certain font style pairings that reflect particular qualities that align more closely with a brand or image that the designer wants to create.
On the other hand, some of these experiences are more objective, for example, you may need to provide different font sizes depending on the screen resolution and device that you want to design for, or even default sizes for different types of users (e.g. larger font sizes for older people for improved readability).
It can also depends on other considerations like if you want to provide support for accessibility, for example fonts that cater for people with dyslexia (but might be harder to read for people without dyslexia).