The CPU has 24 lanes and the chipset can add up to 8 additional lanes, see [1] first table.

It's a pity that they did not include at least 32 lanes on-chip to allow two full x16 GPUs at least on the 1800X, but I hope there is a 1900X in the pipeline with more lanes.

[1] https://arstechnica.com/gadgets/2017/03/amd-ryzen-review/