Let $A$ be the set of students who selected the topic "Selected Lectures on Geometric Proofs", and let $B$ be the set of students who selected the topic "Polar Coordinates and Parametric Equations".

Then, $A \cup B$ represents the entire class.

Let the cardinality of $A$ be $x$, then the cardinality of $B$ is $x + 8$.

The cardinality of $A \cup B$ is 54, and the cardinality of $A \cap B$ is 6.

Since the cardinality of $A \cup B$ equals the sum of the cardinalities of $A$ and $B$ minus the cardinality of $A \cap B$, we have:

$$54 = x + (x + 8) - 6$$

Solving for $x$, we get $x = 26$.

Therefore, the number of students who selected only "Selected Lectures on Geometric Proofs" is $26 - 6 = 20$ students.

Hence, the answer is $\boxed{20}$.