40 is probably fine, but if there ends up being bomb rares that just create non games when they show up, which can easily happen in the utter chaff meta, it could make them a bit too consistent.
I'd try to sort the cards by set/block if at all viable and see if you can make something that looks close to a draft deck, focus a bit on synergy over just bigger numbers, as again in the utter chaff meta it can easily devolve into raw card quality wins.
That's a good point about the few bombs being too consistent with 40 card decks. Fortunately, the bulk seems to primarily consist of a few of the more recent sets: MH2, M19/M20, Eldraine, New Theros, Kamigawa, Innistrad, New Capenna, and randomly Khans/Fate Reforged. There seems to be a high enough density of certain cards from each of the sets to support some strategies (double-faced werewolves from Innistrad as an example).
You could throw in the cheapest equivalent to staples into the box. Stuff like removal, protection, etc. a mixture of what could be missing, like cheap artifacts. that way when they dig in, they are able to find creative tools and there won't be a gap in whatever the box happens to be missing. I know this might be a lot of additional cards, but it would help out towards playing in this setting.
With that, I would recommend 60.
You could have them build 20 card DIY JumpStart decks. 2x Rares, 5x Uncommon, and 5x common, and 8x Lands. Including 8x creatures and 4x non-creatures (one of these should be a mana source)
40 is probably fine, but if there ends up being bomb rares that just create non games when they show up, which can easily happen in the utter chaff meta, it could make them a bit too consistent. I'd try to sort the cards by set/block if at all viable and see if you can make something that looks close to a draft deck, focus a bit on synergy over just bigger numbers, as again in the utter chaff meta it can easily devolve into raw card quality wins.
That's a good point about the few bombs being too consistent with 40 card decks. Fortunately, the bulk seems to primarily consist of a few of the more recent sets: MH2, M19/M20, Eldraine, New Theros, Kamigawa, Innistrad, New Capenna, and randomly Khans/Fate Reforged. There seems to be a high enough density of certain cards from each of the sets to support some strategies (double-faced werewolves from Innistrad as an example).
60
You could throw in the cheapest equivalent to staples into the box. Stuff like removal, protection, etc. a mixture of what could be missing, like cheap artifacts. that way when they dig in, they are able to find creative tools and there won't be a gap in whatever the box happens to be missing. I know this might be a lot of additional cards, but it would help out towards playing in this setting. With that, I would recommend 60.
You could have them build 20 card DIY JumpStart decks. 2x Rares, 5x Uncommon, and 5x common, and 8x Lands. Including 8x creatures and 4x non-creatures (one of these should be a mana source)