It's not a controller issue. From configuration guide, beginning of section "Working with VSCs":
"The MSM760 and MSM765 zl controllers support up to 64 VSCs. The MSM720 supports
16 VSCs by default, and 64 VSCs when the Premium Mobility License is installed. All other
controllers support a maximum of 16 VSCs. Controlled APs support a maximum of 16 VSCs."
Note the limitation on controlled APs - this means per AP group. So if you need more than 16 VSCs, you will need to create another AP group and assign some of the VSCs to the new group.