You can drive up to 3 external displays using expansion cards in the 3 slots with DP Alt Mode.
These are driven off of the iGPU.
As for the dGPU, I don’t think there’s any form of offical confirmation on it’s capabilites yet, I imagine these will come out with the Expansion Bay Deep Dive.
I don’t see any reason why you wouldn’t be able to use iGPU and dGPU at the same time, so there’s the potential to have something like 5 monitors plus internal screen, if you wanted to.