If I stupefy a unit but it's no longer there as a reaction (opponent played retreat or something similar), do I still get the draw?

Yes, you still draw the card.

According to RiftJudge FAQ #669, spells resolve even if some or all of their targets become illegal. While you must have a valid target to play Stupefy initially (as clarified in FAQ #6785 and FAQ #4286), once the spell is on the chain, it will resolve as much as it can.

Sequence of Events:

1. Play: You play Stupefy targeting a unit. It is placed on the chain.
2. Reaction: Your opponent plays a card (like a retreat effect) that removes the targeted unit from the battlefield.
3. Resolution: When Stupefy attempts to resolve:
   • The instruction to "Give a unit -1 might" cannot be executed because the target is no longer on the battlefield (it is now an illegal target). Per Rule 359.3.e.6, this instruction is ignored.
   • The instruction to "Draw 1" does not require a target and is not dependent on the first instruction. Therefore, it still executes.

The spell resolves, the targeting instruction is ignored, and you successfully draw your card.