How many tiles a surface needs, including a cutting allowance.
A rectangular room laid straight needs a 10% margin. Diagonal laying or a room with many alcoves calls for 15%, and large-format tiles up to 20%, since every cut wastes a bigger piece. What is left over also covers later repairs.
Divide the room's area by the area of one tile. A 30 × 60 cm tile covers 0.18 square metres, so ten square metres takes 56 tiles, or 62 with a 10% margin. Boxes are sold whole, so round the final figure up to a full box.
Slightly, and usually in the buyer's favour. A two-millimetre joint on a 30 × 60 tile cuts the count by less than one percent, so it can be ignored. What matters far more is the laying direction and which edge the first full tile starts from.
Adhesive runs at roughly 3–5 kilograms per square metre depending on the notch size and how flat the substrate is, so ten square metres takes one 25-kilogram bag. Grout usually goes about one kilogram to three or five square metres.