This is covered in the documentation here
IP Camera - Bindings | openHAB
With 20 cameras yes I am not surprised that M3 is still an issue, I have not tested that many before but you do not need to wait for M4 as I have uploaded the jar if you wish to try it out.
That is difficult to answer as it depends on the browser and the widget. You can easily check yourself by using DEBUG log output. Sometimes it is needed to press refresh on a browser after you leave the page to stop some streams.
Your streams not starting whilst others do, are any of them hikvision with digest? if so see here…