Specifically, I'm a Zerg player learning to fast expand and trying to play a more macro game. How many drones do I take off my main and put at my expo? Is this number consistent with future expos as well?
There are two schools of philosophy on drone transfer (this only is for drones on minerals):
Both have their merits.
The first is based on the assumption that any unit after the 16th is only doing a fraction of the work it could do otherwise; transferring those means you gain more minerals without any additional cost. Most companies would kill for this opportunity.
The latter is based on the idea that when powering drones you don't want to have to select your Hatcheries individually, so by doing this both sites need just as many new drones and you can power without noticing location. This often falls under the criticism that any drones being transferred aren't producing minerals; this is fine when they're at reduced effectiveness, while it's a bad idea under 16. While this is true, any drones produced at one base and rallied to the next suffer from the same problem, so the net loss is actually zero (unless you don't rally, in which case your economy is slowed for other reasons).
If you start playing with really ridiculous math (ridiculous because of the assumptions you have to make) you should find out that the net effect is the same. You will notice that Pro players tend to favor the former as more money now is better than the same money later.
So, my suggestion is: all drones on minerals but 16.