There are an essentially infinite number of unit cells possible. For example, a square unit cell may contain any of the following object patterns.
S1 unit cell S2 unit cell S3 unit cell
One hexagonal unit cell might look like any of the following.
H1 unit cell H2 unit cell H3 unit cell
Notice that the circles in S2 and H1 are the same. However, the empty
space around the circle - which is also part of the unit cell - is different
in the two unit cells. They will pack together differently; S2 in a square
pattern, H1 in a hexagonal one.