If you ultimately want 8 monitors, you will most likely need 2x, quad-port video cards. (The other option is to have 4x, dual-port cards... but you'd need 4x, PCIEx16 slots on your mobo to mount them.)
IF your mobo has 4x, PCIEx16 slots... you could use something like Nvidia NVS 310/1GB video cards which you could buy used for ~$20 each on eBay. (They may be old, but they work well.) If you don't have 4x, PCIEx16 slots, you'll have to go for the 2x, quad-ports. Possible options... Nvidia Quadro P620 (which I use) for ~$165-$180 each or less if you can buy used. There is also Nvidia Quadro T600. AMD has WX3200 and Wx4100. The P620 will likely be the lowest price. Buy hey.. shop around. Might be some left over Presidents day bargains still going on.
Good Luck!