One symbol or a small constellation
A crescent moon works as a single focal point, while a few scattered stars create a more open composition. Decide which feeling you prefer before adding further elements.
Use empty space intentionally
The space between small symbols is part of the design. Try arranging references on paper to compare a close cluster with a loose trail of stars.
Your style comes first
These ideas are for anyone drawn to celestial imagery. Bring the shapes, placement and line styles you enjoy to your artist, who can turn the references into a piece made for you.