You will find the numeral 9:
As part of a 9-8 suspension. Here the full figure is 9/5/3 - 8/5/3.
As part of a 9/7-8/6 suspension. Here the full figure is 9/7/3 - 8/6/3. Sometimes you will find an extra #5 in this figure, in which case the full figure is 9/7/#5/3 - 8/6/3.
As a functional ninth chord. This is such a rare chord in 17th- and early 18th-century music that you don’t need to worry about it. If you want to try it out, play a functional seventh chord and add a ninth. The ninth has to resolve down, just like the seventh.