Last time I tested this I couldn’t get past 31 streams (that’s with cudacodec which uses ffmpeg to stream from rtsp), maybe its dependent on the version of the ffmpeg libs which opencv is built against. I would try testing the latest version of ffmpeg on its own and if I could get more streams try to build opencv against that version.
Is the CPU near 100% usage? VLC could be using the GPU for decoding, I don’t know about Gstreamer.