Please be aware that in some situations the PCIe slot could have not all the lanes (speed) available.
First of all, find out how many PCIe lanes supports your CPU (16/28/44 - this info should be in specifications).
Then check your motherboard's manual and find information about how many PCIe slots it has and how much speed they can have with different-lane CPUs.
Finally, check that your slot provides enough PCIe lanes (speed) to fulfill your capture card requirements. You can find this information in the card's specifications. Otherwise, there could be performance issues with your capture card like audio and video jitters, etc.
Here is an example:
It is Intel Core i7-7820X CPU, which supports 28 PCIe lanes and ROG STRIX X299-E Gaming motherboard. It has three 16-lane PCIe slots, which can work in different modes depending on the CPU.
In the motherboards user guide, it is mentioned that if you use all the three x16 slots, the last one will provide only 1 lane (x1 speed), which is not enough for, for example, the BMD Duo 2 card. This device requires 4-lane PCIe slot. So you can use the first or the second 16-lane slot for this card or you can use the 4-lane slot (at x4 mode) [PCH] which is shared with SATA ports. You can find it on the previous picture: