Transports do bandwidth estimations etc to adapt to the network conditions. Multiple transports means more calculations per transport which means more CPU. But the CPU increase is not much.
→ Using one transport for all producers in recommended but not compulsory.
→ Same is true for consumers as well but as consumers are more in numbers so usually we divide them over multiple transports like all consumers of one participant on one transport or 10 consumer per transport etc. But you can still have them on one transport no matter how many they are, it is totally possible.
At the end it is up to you how you want to do it. I personally use one transport for one producer, consumer. It helps me with the scalability on server side but that is my own logic yours can be different.
Another approach can be that you use one transport for all producers but for consumers you can use one transport per participant like share screen, video , audio of one participant goes under one transport.
There can be many other approaches.
This is deeply explored here: