In my case isn't "how many can I build" as much as "how many can I stand to build for other people who want an instrument with 4 batteries, 14 switches, 3 pickups, and a tremolo on a body shaped like a woman's breast".
I stopped taking orders for "the custom you design" as it was sucking the life force out of me. I now build what I like, when I like and sell it after that. The basic design has minor options. I have limited my audience with this and so far it's been a good thing. Thinned out the martians.
I'm fortunate to have a good day-job and do this for fun and make some money on the side. I've kept my biz in the black for 9 years this way and still enjoy it immensely. I could probably stand to make 6-8 a year and keep the fun factor in it.
Most builders have repair work to suppliment their custom stuff. I have a large number of local collectors and eBay addicts that keep me busy with wierd stuff. Even there I end up turning down some of the pain-in-the-a$$ stuff to keep my sanity.