How many tons of asphalt in 1 square meter 50 mm thick?

Anywhere between 0.08 tonnes and 0.125 tonnes.

Depending upon the type of asphalt, and its actual make up, the density of asphalt can vary between 1.6 tonnes/m3 and 2.5 tonnes/m3

1000 mm = 1 m

mass = volume x density

volume = 1 m2 x 50 mm

= 1 m2 x 50 ÷ 1000 m

= 0.05 m3

The mass can vary between:

lightest: mass = 0.05 m3 x 1.6 tonnes/m3

= 0.08 tonnes

heaviest: mass = 0.05 m3 x 2.5 tonnes/m3

= 0.125 tonnes