So I did a few quick calcs and I believe I can replicate the damage multipliers closely.

21 total = 6 <suit cards-1>3 <total cards-4>3

<21 = max(total cards-2,1) round(total value/20)*suit cards

Where:

Suit cards = number of cards dealt of one suit

Total cards = number of cards dealt

Total value = value of all cards dealt

<> = If value is below 0 then 0, otherwise use calculated value.

TL:DR multipliers are affected by the number of cards and the total value/20 when less than 21. 21 ignores value of cards (21, obviously) and depends on number of cards.

E:spreadsheet

There is an effect of aces from skills that I don't understand, but it seems to ignore the ace for card counts...