UK commercial brewers will typically add 0.5-1bn cells per litre - towards the lower end for bottles where you're more worried about avoiding "sediment", higher for casks where it's less critical. Given that a quad has much higher alcohol than the kind of beer typical in the UK, and conditions at home may be less precisely controlled, I'd go for 2-3x that. Given that a fresh pack of dried yeast will typically have around 20bn viable cells/g (depending on cell size and response to freezing), you're looking at 1-2g in 20l/5 US gallons.
Personally I wouldn't. Remember these are not hops but living cells, and the great enemy of life is ice crystals. Yes in theory dried yeast should be pretty resilient to freezing, but there's not a massive gain in life in going from fridge temperature to freezer temps, and given the risks associated with the vagaries of home freezers in the real world, I prefer to keep my yeast at home in the fridge rather than freezer, unless as glyerols.