How to use more than 32 downstream subnets for multicast routes (e.g. smcroute or igmpproxy)?

smcrouted (with -N option) works perfectly fine with max. 32 virtual interfaces. With 33 ore more:

daemon.warn smcroute: Kernel MAXVIFS (32) too small for number of interfaces: Out of memory

Please, could you tell me how to increase that limit? I didn't find the responsible place in the kernel menuconfig. I found a similar question on stackoverflow but the answer didn't help.

EDIT: openWRT 19.07.7 is running on a x86-64 machine with 254 VLANs.

Wow. What is this like an entire university campus with 254 buildings?

Just one building with 254 apartments for students. Some of them (more than 32) would like watching IPTV using dlna clients.