Many of the best YC companies have had phenomenally small number of employees for their first year, sometimes none besides the founders.
Keep salaries low and equity high. Keep the organization as flat as you can.
Execution gets divided into two key questions: 1) can you figure out what to do and 2) can you get it done.
Remember that the idea will expand, and become more ambitious as you go.
The best people know that they should join a rocketship.
In YC experience, 2 or 3 co-founders seems to be about perfect.