Impact of jms thread count increase

We have 16 processes consuming from 2 queues, 8 processes are connected to Q1 and 8 to Q2.
Once messages are read from the queues by a java application, they are sent to 2 different servers (S1 and S2).

My question is related to the number of jms.session threads (or any other parameter). Counting that the default value is 10, does that mean that there are 10 threads to serve 16 processes?

If for some reason S1 goes down or responds very slowly, 8 threads are going to be locking up messages in Q1, but more important than that only 2 threads (10 total threads - 8 locked) are going to be available to attend 6 appication.

My conclusion was that we should have as many jms.session threads as consumers connected to one router to avoid deadlocks or throughput problems afffecting S2 when the problem is that S1 is not available. Is this correct?

What is the potential impact of increasing the thread count to 20 for example? Any risk? Any recommended max value for this case?

What is the impact / role of network.session parameter?
Please let me know if you need more details to answer this question.

Thanks

Q-alot