St Tropez cream only has one shade, I used to use that and all my clients were very happy. I think it is important for the colour to be good on all skin shades in this case and you know your clients best, if thy are happy with the depth of colour then why not.
I use Vani-t and although they have a number of strengths I only use two shades as my clients are mainly very pale skinned (which is why they want the tan in the first place). I can see the benefits of having different strengths in some cases but it depends on your client base. There is no point in having a wide range when it will go to waste as all your clients wanting a tan are pale and interesting. :hug:
Don't panic you can change your mind and add another shade from another range if and when it is needed! You can offer different skin care ranges for different clients needs so why not tanning solutions? I know a number of salons that offer a choice. You are the professional so you can make the decision on what is best for your client. Don't worry about what everyone else thinks or does, if you are worried about not being able to service your clients properly with this one shade get another but as they say "if it's not broken don't fix it"x