You'll need a mixer to combine, process and route the audio from multiple Volcas to your headphones or speakers.

The most important feature is the amount of channels - the more channels the mixer has the more equipment you can hook up to it. Figure out how many you’ll need and keep future expansion in mind.

Other beneficial features to look for is at least basic equalization capabilities (allowing you to adjust the tone of each channel by boosting and cutting lows, mids and highs and thus better "glue" the sound of your Volcas together) and aux sends/returns (for applying audio effects like reverb or delay to multiple channels with effects pedals).