I turned mine into a still

A touch cheaper on Amazon but you don't get the tap or the airlock
(Oil Drum Holds 50 Litres Made of Stainless Steel Wide Mouth Container with Tap Opening
https://www.amazon.co.uk/dp/B00GXNA5AS/ ... Qxb4CMSVRH)
Yeast will multiply until their numbers find an equilibrium with what's available in their given environment, if you add a little then in time the colony will grow or you add loads & the colony is already there. Add too much & my understanding is that some will die off but live yeast will use dead yeast for nutrient so no big deal. If you're doing a big wash it's just not worth the risk or the time for the sake of probably the cheapest ingredient, hoof it in there!
If you use 1 turbo in 20 litres then 2 sachets in 40 is no different but if you only use 1 then they will need more nutrients to support a colony growing into a 40 litre wash.
I await Robert to contribute a recommendation of a kilo of bakers yeast that's done in 12 hours after sweeping the foam out of his kitchen door!!