good luck in your enterprise.
do you have space to exercise the "boarders"?
May I suggest you tell the owners the exact total price they will pay on return because I have seen arguments when owners calculate wrongly and it saves any ill feeling, also ask the owner to sign that if the dog needs to see a vet they will be responsible for the bill not you.
once again good luck, the lady that boards my cat started as you with 2 boarders and now can take 12 cats and very well looked after too., so hopefully you will expand in time.