Number of GPUs you can get out of motherboard is easily found by looking at 3 slots. PCI-E 16x, PCI - E 1x and M.2.

You add all those together and that gives you total number of GPUs you can attach and get working with that motherboard. There are some limitations for OS without modifying it ( Windows no more then 8 GPUs ).

As example, two best selling motherboards from amazon.com

ASUS PRIME Z270-A - 3x PCI-E 16x, 4x PCI-E 1x and 1x M.2 thus allowing you easily build 8 GPU or less GPU rig with it.

ASUS ROG Maximus IX Hero - 3x PCI-E 16x, 3x PCI-E 1x and 2x M.2 thus same as PRIME Z270-A total of 8 or less GPUs.

So basically, get cheapest motherboard that have sum of those 3 slots to number of GPUs you want to attach.

Just keep in mind that you will have to change some BIOS setting to make this work. One reason specialised motherboards like Biostar TB250 or Asrock BTC+ are easier - BIOS already pre-set to see all possible GPUs you attach.