How many 450mm x 450mm slabs in 40m2?

It depends on the shape of the 40m2. If it is 1m x 40m then, despite the area being 40m2,

there is not a single 450mm x 450mm in it. The shape is simply not wide enough.

But if you're allowed to cut up the slabs to make them fit, then you need 198 of them

in order to cover 40 m2 of area.