I have the be quiet! power 9 psu and the asus prime b450-plus motherboard I'm using a external graphics card and a NVME SDD so I guess it'll use 2 PCIe ports.
The manual says I have to plug in a 8 pin EATX +12V power plug. Now my question is: I have a 6 pin cable with a 2 pin cable, described with PCIe and 12V1, which do not fit. And I have two 4 pin cables with the description of CPU. Can I also use the two CPU cables or do I have to buy a different PSU?
The cable your motherboard is requiring you to plug in is an 8-pin CPU/EPS ATX power, which is also called EATX12V, connector. This cable, depending on your PSU may be composed of one single 8-pin cable, or two 4-pin cables designed to be connected side by side.
You may connect the two CPU power cables into the EATX12V, and they should enter effortlessly. Do not force them in.
On the other hand, the other connector you are talking about is a GPU 6+2 pin power connector, which is designed to be plugged in to your dedicated GPU. This connector is not compatible with EATX12V. Do NOT plug it in there.
You may reference the pinout with the manuals, or the below provided picture:
