The more notes you add to chords, the more difficult they are to play and the more muddy they can sound. For this reason it is common to omit notes from extended chords, keeping only the notes which most clearly indicate the quality (sound) of the chord.
The most frequently omitted note is the 5th, although sometimes the 3rd or the root can be omitted. Usually the bass will be playing the root note anyway. With a dominant 7th chord, as long as the b7 degree is in the chord you still get the effect of a 7th chord. With a dominant 9th chord, as long as you have the 3rd, b7th and 9th, you have the effect of a 9th chord, etc. Shown below are some common voicings of various 9th, 11th and 13th chords, along with some examples demonstrating their use. The first two are Major and Minor 9ths.
Cmaj9
Am9
Here are some commonly used dominant 9th chord (usually just called 9th chords) shapes and an example making use of them.
E9
E9
E9
E9