How many of your client are full-time writers? Excluding the Tom Clancy/John Grisham sorts who own most of the publishing GDP -- and writers whose spouse has a good job to make the BMW lease payment and keep the gin bin well-stocked -- how many writers are able to support themselves (and live in something slightly more elegant than a cardboard box) solely off their writing?
Not many. And it's not the money, it's the health insurance.
I have 6 full time writers. About 1/4 of the active list.
I have no idea if that's average or not.
One of the more interesting accounting practices is figuring out where most of your income is: if I have more than 20% of my income coming from any one writer in a year, I know I need to get cracking. It's the equivilent of putting all your gin in one bottle.