Monoprice 10 channel. Its like $80. All mono. Plenty of channels for all the Volcas.

Monoprice 10-Channel Mixer with USB https://www.amazon.com/dp/B011JNVNVQ/ref=cm_sw_r_cp_apa_i_RWFGCbTNBPJMX

Also buy 3.5 to 1/4" cables.

https://www.perfectcircuit.com/lmntl-12-3-5mm-to-1-4-adapter-patch-cable-13.html