How many brands use own drivers?

Magico is iffy. While the cone materials are custom, they rely on Seas and Scanspeak to make them, and the motors are based on existing units.
I'd call them more "extreme custom" than in-house drivers.

A more average custom driver is what Wilson may order. Off the shelf driver with custom parameters.

Monitor AudioYamahaTechnicsJBL

BOSE !!!!!!!!!!!!!!!!!!!!!!!!

BTW, I make no quality statement about who uses in-house drivers and who does not.
From the manufacturer, in house drivers is a HUGE cost savings, and removes your product from being perceived as a pure commodity. However, that doesn't mean your product is any better or worse than others, generally.

Gryphon and Wilson audio for instance make fabulous use of mostly off the shelf drivers.

Bose is a fabulous example of an in-house driver as well as incredibly tight alignment with marketting and consumers.

They achieve the magical goal of being extremely inexpensive to produce while giving consumers enough value to merit ridiculous prices for a kitchen radio.