Only one AGP card.
Strictly speaking, in certain cases like the i815E chipset, you have the onboard i752 video and an external AGP card. However, you can only run one at a time, and not both.
AGP is a PORT, not a bus, meaning that it can only handle one device at a time. Certain cards like the 3dfx Voodoo 5, one of the coming GeForce 4 MX cards with dual GPU, use an AGP bridge chip to allow two video processors....but this is still a single card.