In tests using a serif face, good comprehensibility was 0 percent, fair comprehensibilty was 12 percent and poor comprehensibility was 88 percent. Use of subheads 78 percent of those surveyed by Wheildon said that they found subheads useful, especially in long documents. Full stops at end of headings Research indicates that full stops have no place in headings. They encourage people to stop reading and pass on to something else. Mixing typefaces can make the job look like a dog's breakfast.
Most typographers stick to two typefaces in a book or report.
Many choose to use only one, but to use it in different sizes and weights. We have always worked on the rule of thumb 'no more than three typefaces, and preferably two, to a document'. We haven't uncovered any research to either support or counter this. We suspect it is a matter of visual elegance rather than comprehensibility. That said, experience tends to suggest that too many typefaces on a page is distracting and may therefore counter comprehensibility.