How Many Rays to Use
The number of rays, especially the ones called late rays, will determine how accurate the results are. However, it will also extent the calculation time. There is not correct answer in how many rays you need. It all depends on the accuracy you need, the time you have, the complexity of your room, if the distribution of sound absorption is uneven and more. However there is some tips in following videos.
Room Setup Part 1: Basics
This video covers the basics in room setup. It is important when using ODEON to be able to set reasonable values for the late rays and impulse response length.