There are a few things you need to consider when getting mallets.
How many octaves of bells do you have, since most mallets are designed for a specific range of an octave or less. Mallet heads get smaller and harder the higher the pitch of the bell.
Second, how many ringers are playing for any one piece. You may need two mallets per person for some pieces that require some fast playing. This can also make easier for younger or older players to play fast repeated notes more consistently.
A third but less common issue is the using different mallets in certain pieces for different sounds on the same bell. This is not too common, but it happens at times.
Lastly, since bells are often purchased in common set sizes (1 octave, 3 octave, etc.) there are similar sets of mallets that are sold as a set for a given common bell range. If you've cobbled together your bell set, you may be able to buy a mallet set that matches most of what you have and then add a few to fill out partial octaves of additional bells.
Note that mallets are far more frequently used in lower registers (around C5 and lower) than in the upper registers. Conversely, bell tree setups (which are always higher octaves around C6 and up) require mallets.
Any good set of mallets have typically color coded heads for their range, which is also written on the handle of the mallet (e.g. G4-C5)
I hope this helps.