This message was posted on behalf of Intel Corporation
I understand you are having some questions about the maximum PCIe lanes that our Coffee Lake processors can provide.
Let me share with you that the lanes that a processor provides are dedicated to graphics so if you install two graphic cards and each one will require 16 lanes the processor might only recognize one. If you are looking for a configuration with two graphic cards I will suggest you to check our Intel® Core™ X-series Processors which will allow more than 16 PCIe lanes. Please find here the link to look for these processors: https://ark.intel.com/products/series/123588/Intel-Core-X-series-Processors
In the other hand, indeed the motherboard should provide PCIe lanes nonetheless, they will not be able to support graphics and, the purpose of those will be for any other task like USB 3.0 or Wireless card.
I hope this answers your questions.
In addition of the previous reply, you can use 2 (or more) video cards in a i7-8700K system.
Just know that the x16 PCIe slots you would be using will be running in a x8 PCIe mode and if more then 2 slots (PCIe 3.0) are used they will be running in a x4 PCIe mode.
This should also be mentioned in your motherboard manual and which slots to use (Mostly from close to the CPU and downwards depending on how it is fitted in it's case.
As Intel mentioned the Core-X CPU's have have enough PCIe lanes (44) to make full use of the video cards (max. 2 = 32 lanes = 12 lanes free) bandwidth / speed.