How much are you selling right now? What are you selling the most of? How often do your customers return? What does your curing rack look like? How many folks from afar are willing to buy your soap online? What kind of soap are they asking for?
Inventory management is one of the toughest parts of having a business...space is money. Soap isn't perishable so it's not going to hurt to sit around for six months, but it's also less space that you have for stuff that does sell. You have to remember, it takes time (six weeks) to cure soap.
To be honest, it's going to be a bit of a guessing game for the first year and you'll undoubtedly have too much of one thing and not enough of the other. Starting out with a dozen bars of a dozen selling scents with backup of each in various stages of curing for the next 6 to 12 weeks weeks is probably not a bad start and then you can start adjusting from there.
A note about your insurance. It might cover you as a tattoo artist, but it's not going to cover your soap. It like having a homeowner/renter's policy...it will cover your vehicle while it's parked at home, but it's not going to cover it while it's on the road. You will need product liability insurance.