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.
 
 

Unit Cells

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