Hi Chris,
You can start with anything really. Main thing is you have enough of a bank to cover your liabilities on the exchanges. Maybe check out the blogs and get an idea about what you’d need.
You can always start small. Only thing that would hold you back is you’ll have to pick and choose your offers and might miss out on some big value ones because of a small bank but that doesn’t mean you won’t be able to build your bank with what you have, it’ll just be a slower process.