I think Asus list their spec slightly different that normal.
What you'd have is 6xSATA3 ports that you can populate & 1xM.2
You also have SATAExpress availability, but this is where the confusion starts.
To populate a SATAExpress drive, it'll take not only it's unique connector, but also 2xSATA3 ports too!
Most manufacturers tend to list the total number of SATA3 ports and then start taking them away (ie listing 6, but then explaining 2 will disable if you use SATAExpress). Asus try to simplify matters by showing the maximum input you can use at one time.
The problem they have by doing this, is the go against the setup the rest of the market uses and start to confuse consumers.
Here is a pic of the board:
The SATAExpress is the 3 ports on the right side.
A single SATAExpress drive will use all 3 ports to run.
If you're not using SATAExpress, you can use the two ports on the right side to use standard SATA3 connections.
This is a SATAExpress cable:
I believe M.2 used to disable SATA3 ports if used also, but not sure if that still holds true?