I use a ASUS Prime Z390-P Motherboard for 6 GPU. Currently i can't use more than 4 gpu at the same time. Maybe i have to change some settig in the Bios.
ASUS TUF Z390-Plus Gaming (Wi-Fi) LGA1151 (Intel 8th and 9th Gen) ATX DDR4 DP HDMI M.2 802.11 AC Wi-Fi USB 3.1 Gen2 Motherboard, Black : Amazon.co.uk: Computers & Accessories
PRIME Z390-A
ASUS PRIME Z790-P WIFI DDR5 Socket LGA 1700 Motherboard