You will need to know the square footage of the room in which you plan to lay adhesive vinyl floor tiles. Tiles are 12" by 12" and come in varying thicknesses. Don't go for the very thin tiles, as they will wear out quickly.
Once you know the square footage of the room, you can estimate the cost and number of tiles needed. It's a good idea to purchase a few extra tiles in case of mistakes. For example, if you have an 8' by 10' room (80 square feet), buy 90 tiles. Then you have 10 extra tiles to work with.