I wanted make a big squishy D20 as a gag for my DnD group, so I made a big squishy D20. Then I wanted to get rid of leftover silicone, so I also made small ones.

The big D20 mold was made to require just under one liter of silicone, since that way there wouldn't be too much left over from a 1l kit, but it definitely wouldn't fall short either. It needs around 950ml, which puts the result very close to 1kg. It's very fun to play around with and has some nice heft to it. The numbers are fairly easy to read, so it can actually be used to roll (or rather bowl) your barbarian's attack rolls.

The small mold yields an approximately standard-sized D20, given the tiny details though, it is hardly more than a novelty item; an FDM printer can resolve the model well enough, that the numbers can be read, but it can be tough. The small D20 uses less than 5ml of silicone a piece.

The D20s' numbers are arranged like on a standard D20, not spin-down. Both molds are keyed, so the arrangement cannot be twisted where the mold halves meet. These keys are are also used for de-molding; there's a gap all around each mold for prying with a flathead screwdriver, with larger gaps at the keys.

For single use molds, 2 walls and 4 bottom / 5 top layers should do, but you're likely to damage the molds when prying them open. That there silicone likes to hold on, and it's got a lot of surface area to hold on to.

As for the silicone: I have only made ones using Shore 20A silicone so far, and I think it's a decent middleground between squishyness and still being rollable. Also, the softer the silicone, the more it turns into a dust magnet.

STEP files and the Fusion project are available if you want to tweak things (though the fusion project is a bit messy).

Edit: IMPORTANT Since filling the big mold through the small hole can take multiple minutes, I advise against fast-curing silicone

Edit 2: Added size reference images