Kastrup: Idea of the World

[Laird]

Can you give the reference where BK says there's only one experiencer?

This isn't the best reference because it's in the context of something else (free will), but for a start, try page 156 of the Kindle edition of Why Materialism Is Baloney (emphasis mine):

I contend that freewill proper is the primary cause of all movements of mind; the freewill of the one subject of all existence.

Somewhat better is this quote from a few pages earlier, on page 153 (again, emphasis mine):

Since the subject of all experiences is the medium of mind itself, it too only becomes actualized in the form of experience: the one universal subject exists only insofar as the experiences it has.

And even better is this quote from page 198 (again, emphasis mine):

The conclusion of this exercise is that our inner sense of ‘I’ is fundamentally independent of any story we could dress it up with. As such, it is entirely undifferentiated and identical in every person. It is formless. This undressed, naked, ‘amorphous I’ is inherent to the membrane of mind at large, the sole subject of existence. Not only does every person have the same inner sense of ‘I,’ I contend that every conscious being has it: cats, dogs, fish, etc. At the deepest, narrative-free levels, they must all feel exactly like us.

No doubt, there is an optimal quote where this notion is introduced in the first place, but it doesn't seem to be in my highlights. Sorry about that.