i am new to amd world so when i Render with my Ryzen 9 3900x it uses 100% of the CPU power which increases the Temp....so i wanna know how do i set the usage of the CPU to a certain percentage ?
You can set a TDP limit with the BIOS. 3900x has 105 watts TDP. If your motherboard allows for over this TDP, even the most powerful coolers may not be able to prevent high temperatures by encoding.
So you need to regulate TDP, not processor usage. You can set the TDP limit to 95. Most cooler are manufactured according to this TDP.
Could you please upload screenshots while benchmarking and idle with information on your build?