I'm solo for now so can't give you DPC group experience but from my experience in third party paid, FFS group practice probably 3-5 docs is the sweet spot. Many more than that and you need to add management staff or layers and it gets complicated.
Size has some advantages though. Cross covering vacations is even easier (although not a problem for most solo docs). Docs in the group will have different skills and expertise that makes the group smarter and better for patients than any one doc. With enough patient volume, hiring or training a rad tech becomes worthwhile and inexpensive plain films in-house become another service that patients value.
How to split up overhead requires some thought, especially how to hand fixed costs vs costs per patient or costs per physician, but is no different from insurance paid group practices.
Most DPC patients prefer seeing their own doc most of the time so that would impact how much you'd want to share patients.
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