i.e. for 6 cards add this to config line, 1024 is the instensity and 8 the worksize: --opencl-platform 0 --opencl-devices 0,1,2,3,4,5 --opencl-launch 1024x8,1024x8,1024x8,1024x8,1024x8,1024x8

And if you repeat the devices, i.e: --opencl-devices 0,0,1,1,2,2... you can use 2 threads per card

Also exists a line / file generator https://config.xmrig.com/