For brush making, how deep?

Hey, so for a 24mm knot I need a 24mm hole wide, but how deep should it be?

I searched but find it quite difficult to find an answer, perhaps someone can tell me here.

Thanks!

12mm will be enough.

Ok so I have the handle now and the hole in it. I also noted to use epoxy glue to glue in the knot, I have that as well. But is there anything else used besides the glue, I don't know, to seal of the entire hole? Not sure, could use help on this one.

So I have, the handle, I put varnish on it that is for outdoor and wet use so I guess that's ok, and I have epoxy glue.

if your brush is made for 24mm knot you should not have any gap to fill.
If you mean the knot does not go all the way down the hole, then put extra epoxy to fill the extra space.
Make sure to put some masking tape on the knot so you don't accidentally mess it up with epoxy.