Density

Normal planet density in Stars! is approximately one planet for every 5000 l.y.^2. That is, if you divided the universe up into squares 70 l.y.^2. on each side, there would on average be one planet in each square.

Selecting Sparse decreases density to approximately one planet for every 6500 l.y.^2.
Selecting Dense increases the density to approximately one planet for every 4000 l.y.^2.
Choosing Packed increases the density in number of planets by 30% over Dense in all but the Huge universe (which is stuffed as full as is playable by the Dense setting).

How many and how dense are the stars in various universe setups?

Density
Normal planet density in Stars! is approximately two planets for every 10000 l.y.^2. That is, if you divided the universe up into squares 100 l.y. on each side, there would on average be two planets in each square.

Selecting Sparse decreases the density to approximately 1.5 planets for every 10000 l.y.^2.
Selecting Dense increases the density to approximately 2.5 planets for every 10000 l.y.^2.
Choosing Packed increases the density to approximately 3.75 planets for every 10000 l.y.^2.

However, the combinations Large/Packed, Huge/Dense, and Huge/Packed have reduced density (most noticeable for Huge/Packed), likely because of engine limits on the number of stars allowed.