In Chaos Marauders and many other units who started out in WFB's case you can have as many banners and drummers as you can fit in the unit.
Be warned though, that units many of this nature with recently updated warscrolls, like Daemonettes and Clanrats are now only able to make 1 in 10, or 1 in 20 models standard bearer and musician, so you might want to future proof yourself and only build one command group per unit!
Double Misfire's post in Chaos Marauder Special Models was marked as the answer