Nvidia pascal focuses on 2

At the official launch of the GeForce GTX 1080, Nvidia announced that it will focus especially on offering maximum performance in 2-way SLI configurations as it is the solution that offers the best performance scaling when using more than one graphics card on the same device..

Nvidia is betting everything on the 2-way SLI in Pascal

Nvidia has introduced a new SLI bridge with the GTX 1080 that focuses on achieving the maximum bandwidth possible in 2-way SLI configurations. This makes further performance scaling possible but the new SLI bridge has the drawback that it occupies both SLI touchpoints in 2-way SLI configurations.

However, you can still make 3-way and 4-way SLI configurations with the GTX 1080 but in this case it is necessary to use the old SLI bridges and special software that makes it possible to use the old bridges on the new Pascal cards.

The movement of Nvidia has logic because the 2-way SLI configurations are the most used by users for their balance between price and performance. Adding more than two cards in the same equipment undoubtedly offers better performance than using only two, but the escalation is much less so much of the price / performance ratio is lost. Given this, Nvidia has decided to optimize its drivers to take full advantage of the 2-way SLI configurations.

On the other hand, let's not forget that DirectX 12 allows to use multiple graphics cards natively, without using the SLI bridges and being able to even combine Nvidia and AMD cards.