All conferenincing systems have a limit on the number of first-class participants (people who can listen and speak). Broadcast is a different problem and there are a ton of really good solutions when you have less than 250 speakers but many listeners.

There are no conferencing products, to my knowledge, that support more than 250 first-class participants over the internet in a reliable way.

If you have an example, I'd be very curious to play with that product.