Hi Ann
A quick dive into
https://www.cheesemaking.com/shop/thermophilic-ds-culture-5-pack.html shows that the manufacturers of this culture recommend it for hard Italian cheeses. Comparing the contents with Caldwell's Mastering Artisan Cheesemaking would seem to confirm that this is appropriate.
As far as amount goes, I tend to work to the manufacturers recommendations which, in this case, is one packet per 2 gallons of milk. Now I think that for those of us who use metric, 2 gallons is about 8 litres and 4 gallons is about 16 litres.
Can you stretch that to 20 litres? I don't know but my guess would be probably. If you did, you could consider extending the ripening time a bit. Alternatively, you might want to reduce the size of your make. Either way, good luck with it.
(Incidentally, and this might be getting a little pedantic, but the Italian word pecorino is used for cheeses made from sheep's milk. The corresponding term for Romano made from cow's milk is vacchino.)