Step 1: Picture the beam and count the supports.
A propped cantilever has a fixed end (locks vertical movement, horizontal movement and rotation, so it gives 3 reaction components) and a prop or roller at the other end (gives 1 more reaction, since it only stops vertical movement). So the total number of unknown reactions is $r = 3 + 1 = 4$.
Step 2: Compare this with the number of equilibrium equations available.
For any plane structure we only get 3 independent equations of statics: $\sum F_x = 0$, $\sum F_y = 0$ and $\sum M = 0$. Any reaction beyond these 3 cannot be found from statics alone and is called redundant.
Step 3: Subtract to get the degree of indeterminacy.
\[ D_s = r - 3 = 4 - 3 = 1. \] This one extra unknown is exactly why solving a propped cantilever needs one compatibility condition, usually zero deflection at the prop.
\[ \boxed{1} \]