Agricola Family Growth by 2 (or more)
You can add at most one newborn regardless of the number of empty rooms you might have.
The rules are clear on this:

After Family Growth, also 1 Minor Improvement (Stage 2): A player may only use this Family growth space if he has more empty rooms than Family members.
A player who chooses a Family growth action adds their newborn offspring to the Action space
In the Return home phase, the new Family member is taken home ...  A Player who takes the Family growth action will therefore have one additional (adult) Family
member to use from the following round onwards.