Limiting spawn of FFMpeg to avoid tvheadend overload

I've a m3u list in which a call ffmpeg in pipe for each channel. Now, I noticed that when for instance I have more than 20 new channels
and I start a scan, tvheadend starts spawning a ffmpeg for channel and after a while tvheadend becomes unreachable and I have to restart it
since all spawned ffmpeg overloads it.

I have already limited the Maximum # input streams to 1 but this was not enough.

Any other suggestions? Tnx