Best thing to do is to speak to your hairdresser, she will know best what shade to go for, going by the colour of your hair just now.
It is also partly determined by your hair itself, it took me 4 trips to the hairdressers to get the colour i wanted, and if your main image is your natural hair colour then you may need more as you are a lot darker than me. Toners etc obviously can be used, but when you're stripping your hair of its natural colour then it does pretty much decide what it wants to do on the first couple of visits. Also i would advise having your hair pre lightened first if you are going for the whole hog, it will make it a lot easier. Just make sure you stock up on the best of the best of the best conditioners etc as you are going to quite literally kill your hair!
Best of luck either way - make sure we get to see the results!!
Apologies if this is a daft suggestion (I'm an old bloke with not too much hair to worry about, so I'm not clued up on these things), but would it be practical to try on a few wigs so that you can get a preview?
My hair has been dyed black since I was 12, its currently faded purple atm. But the roots are showing as a sort of medium brown.
Yeah I just need to find a new hairdresser I like (i.e can actually do their job and gets my random sense of humour ^^), as I'm stupidly fussy about people near my hair and my old hairdresser moved to Malta a few weeks ago