8. If five vertices of a regular nonagon are colored red, then there are at least $\qquad$ pairs of congruent triangles (each pair of triangles has different vertex sets) whose vertices are all red.
8. 4 .

A triangle with both vertices colored red is called a "red triangle". Thus, there are $\mathrm{C}_{5}^{3}=10$ red triangles. For a regular nonagon, the triangles formed by any three vertices are of only seven distinct types (the lengths of the minor arcs of the circumcircle of the regular nonagon corresponding to the sides of the triangles are
$$
\begin{array}{l}
(1,1,2),(1,2,3),(1,3,4),(1,4,4), \\
(2,2,4),(2,3,4),(3,3,3)
\end{array}
$$

assuming the circumference of the circumcircle of the regular nonagon is 9$)$.
If there are at most six distinct non-congruent red triangles, by Turán's theorem, there must be at least four pairs of congruent red triangles; if there are all seven distinct non-congruent red triangles, then particularly, there must be those corresponding to the minor arc lengths $(1,1,2)$ and $(3,3,3)$. Thus, essentially, it must be one of the two scenarios shown in Figure 5, but neither of them contains all seven distinct non-congruent red triangles, leading to a contradiction.
Therefore, there must be at least four pairs of such triangles.
Any of the scenarios in Figure 5 can serve as an example when there are four pairs.