How many slump tests must be carried out for 20m3 of concrete?

Frequency of Slump testing is carried based on project specifications, however I would recommend  following depending on where you are !

In your case one test should be good or  one test for every transit mixer you receive. Transit mixer comes in 8m3 capacity.

At site receiving  end of concrete :

1. One test for every 50m3 concrete placed or very time you cast cubes.
2. If you find variation in workability perform a test.

At batching plant :

Slump test is carried at every 1000m3 concrete produced  per grade.