Maggie - you need to work out the square meterage - e.g. length times height. and buy the number of packs indicated. There tends to be less wastage with mosaic tiles as they are so small.
Mosaic tiles are a pain to apply and even worse to grout. If the wall isn't perfectly flat they look terrible. Have you tought about glass splash backs. V cool e.g.
http://www.amazing-glazing.com/cglass.html
And no, I've no connection with the company. Picked at random.