On a low-end PC, it is unlikely to see more than 2 empty slots, let alone 3. And extremely challenging to have 4 to 6.
Driving 4 additional monitors is possible, through some of the USB-to-DVI/VGA/HDMI adapters. They work okay. Some nuisance because of using USB, but does the job for the most part.
Running many video cards can be an issue to the power supply, even though they are low-end video card. Many low-end PCs are equipped with marginal power supply - only enough for the main motherboard and one or two disk drives. Any additional load may need to change out the power supply too.