Is there a way to find out how much is decoder used

I checked Jetson AGX Orin NVDEC max frequency which is 998 Mhz.

sudo cat /sys/kernel/debug/bpmp/debug/clk/nvdec/max_rate

When I run a gstreamer pipeline with 1 or 4 rtsp sources I would like to understand how much is decoder used, But NVDEC is always showing the same number - 998MHz. Is there a way to see what is the NVDEC load?

Note that you are querying max rate, not current rate. Better try:

sudo cat /sys/kernel/debug/bpmp/debug/clk/nvdec/rate

You may also get some activity monitoring stats from sysfs:

sudo su
ls /sys/kernel/debug/nvdec/
cat /sys/kernel/debug/nvdec/actmon_avg