Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thank you for sharing your questions about vinyl tiles.
Sometimes the coverage area is stated on the side of the box. For example, one box will have five tiles inside and cover an area of 1.2 sq2 meters. All we need to do is to divide 18 sq2 meters by 1.2 sq2 and we get an answer of 15 boxes.
To break it down even further if the tile you have is 474mm x 474mm it would have a coverage area of 0.2247 sq2 meters. In order to cover one square meter, you would need five tiles which is equivalent to 1.1233 sq2 meters. You then divide 18 sq2 by 1.1233 sq2 which gives you an answer of 80 tiles.
I often recommend getting one extra box for errors and accidents. It is also handy for future repairs in case something damages the floor.
It sounds like a very interesting project. Any updates you can provide while re-laying your floor would be much appreciated.
If you need further assistance, please let us know.