How to get number of contact pair which simulator is going to check

[ShangqunYu]

Hi, I am using mjx to train a agent to do some loco manipulation task. The sim is very slow and I think I need to decrease the number of geom that can cause contact in the simulation. I can use model.nexclude to get number of excluded pairs. How do I get number of pairs that will be considered by the sim? Is it model.npair? But it returns 0
Thanks for the help🙏🏻