Q: What keys should I buy?
A: Diatonic harmonicas come in at least 12 different keys corresponding to every available pitch of the chromatic scale, some which are repeated an octave higher or lower. Some keys are more useful to have than others. A good starting set are the keys of C, G, A and D,. esp. if you play along with guitarists who favor open chords. If you are more apt to play along with horns, the flat keys come in handy, namely F, Bb, Eb or Ab. As you become familiar with playing different ~positions~ (see below), the question of ~which keys~ will become easier.
Chromatic harmonicas usually come in C and G. Some are available in Eb and other keys. Since every note is relatively easy to obtain on a chromatic, and since they are usually used as single note instruments (linear/melodies), playing positionally in any of the keys is much more common than on a diatonic. Hence there has been perceived to be less need for as many different keys as there are for diatonic (among other reasons) -- HA.