How many 20 x 45 cm tiles in a square meter?
If you mean a square meter as in one meter (100cm) on each side, forming a square boundary, then you are limited by 100cm in each dimension. You can go 5 tiles wide (100/20=5) and only 2 tiles tall (100/45=2 with a remainder of 10cm). Multiply #wide by #tall to get 10 tiles. If you’re speaking a square meter as in any shape that makes 100cm*100cm=10,000cm^2, then by all means listen to the other people and not me.
