20. A convex quadrilateral paper sheet has $n$ points inside it. Together with the 4 vertices of the quadrilateral, there are $n+4$ points, and no three points are collinear. Using these points as vertices, the quadrilateral is cut into several triangular paper pieces. The maximum number of triangles that can be cut is 1992, then $n=$
20. Let the maximum number of triangles that can be formed be $a_{n}$, then it is easy to know that $a_{1}=4, a_{n+1}=a_{n}+2$,
$$
\begin{aligned}
\therefore & a_{n}=a_{1}+2(n-1)=2 n+2 . \\
& \text { Let } a_{n}=1992, \text { we get } n=9955 .
\end{aligned}
$$