I have booster drops. I have 8%, 10%, 12% and 14% solutions. As 14% is the highest my solution goes, i have booster drops for those clients who want a darker tan. I wouldn't just buy an 8% solution and booster drops though.
I would definitely have an 8 and 10 as these are probably the most popular, then get some booster drops and see how many clients want a darker tan. Once you have ascertained what your clients want, you can then get the solutions you need but definitely get an 8 and 10 to start with.