thanks for responding and I have found this on their site, but I dont know what this means -

PITCHING: 80 to 120 g/hl for fermentation at 12°C – 15°C (53.6-59°F).

increase pitching for fermentation lower than 12°C (53°F), up to 200 to 300 g/hl at 9°C (48°F)

thus my questions -

How do I get from gallons and gravity to metric something or another?

I just did the math and confirmed 4 packs.

100 g/hl = 1 g/l

There are 3.79 l/gallon and you have 12 gallons, so you need 45.4 g.

There are 11.5 g/packet of dry yeast. So, you need

45.4/11.5 = 3.95 packets of yeast.