If Sony sell a Pro, they've sold a PS4.
The Pro is a beefed up PS4, not a separate product line. Sony don't care if you buy a Pro or a Slim, they want you to buy a PS4. There is no cannibalising.
The individual sales numbers for the Slim and Pro don't matter unless the Pro is a success. If this thing sells, Sony will not have a problem releasing individual sales numbers for both consoles. If the Pro sells poorly, they will lump the sales together to mask the failure. That's what Microsoft did. MAU was all that was important until 3 months ago when the Xbox One S became the best selling console in the US. Now they can't shut up about hardware sales.