How do doubled letters work — does "mummy" encode 33 or 333?

Count sounds you actually say, not letters: a doubled letter pronounced once = one digit.

• mummy → you say /m/ twice (mum-my) → 33.
• muddy → one /d/ sound → 31, but midday → two separate /d/ sounds → 311.

So it's about audible sounds. A double letter that's pronounced as a single sound counts once; two distinctly pronounced sounds count twice.