I am unable to recommend any particlar store but will be happy to check your purchases for quality, quantity and fit. I will also advise you on the correct adhesive.
Estimating quantities:
One square metre is 1m X 1m this equates to approximately 10 square feet or 10 tiles of 30cm X 30cm.
Some boxes of tiles can be 9 tiles at 33cm X 33cm each ... also approx. = 10 square feet.
Other common sizes are 10cm X 20 cm, 10cm X 10cm, 5cm X 10cm edging.
Number of tiles required = area required to be tiled (square cm) divided by tile size (square cm)
Lets look at an example of an area for tiling of 8 square metres (or 80,000 sq cm) divided by a tile size of 10cm X 20cm (or 200 sq cm)
so 80,000 divide by 200 = 400 tiles and then add 10% for cutting and wastage etc so total required = 400 + 40 = 440 tiles.
Tiles left over would normally be returned for a refund with your receipt unless they are specifically end of line "sale" items. Check with your supplier.