How many brands use own drivers?

I think you have to understand why they build their own. Some do it because they need to drive the price down on certain products or product lines.  Some do it because they build so many different types of speakers its easier to build these slight variance in models on the same machines (JBL, Focal).  Some do it for performance reasons and the OEM part they need is not available (ATC).  Once you understand that, then the next thing to know what is the QC procedures applied.  Many manufacturers have a +/- 10dB "pass" windows (although few would admit it).   This is not a super high end performance driver, but it could be an acceptable live sound high power driver.  Some of them have smaller pass windows, like +/- 6dB or they don't really test them under realistic loads.  I would expect almost none of the OEM or in house speaker manufacturers use +/- 2dB QC windows.  Few of their customers have a reliable way to measure anything to that small a QC window and wouldn't really know if it was that tight or not.  It requires substantial investment in measurement gear and facility to pull that off in manufacturing or incoming QC over and over all day long.  
Brad  

Wilson Benesch has their own house drivers, though I think they use a ScanSpeak tweeter.

And doesn't Sonus Faber also have their own drivers?
Decware does for sure.

To clarify regarding Wilson Benesch, their top Geometry series speakers use entirely custom drivers including a unique in-house designed tweeter.

Paradigm.  Totally vertically integrated...they design and manufacture their drivers, cabinets, etc.

As a point of clarification many of the companies listed use their own driver designs, but not all actually manufacture the drivers themselves. Many companies use existing driver manufacturers to build custom drivers to their specs.