How to explicitly split long TOC in beamer?

My suggestion is avoid split the ToC in several frames. Alternatives:

1) Start only showing the sections (without subsections):

\begin{frame}{ToC}
\setcounter{tocdepth}{1}
\tableofcontents
% \setcounter{tocdepth}{2} % allow subsequent ToCs *with* subsections  
\end{frame}

... or shorter:

\begin{frame}{ToC}
\tableofcontents[hideallsubsections]
\end{frame}

Simplicity is power in a presentation.

2) Not incompatible with the previos point, show the ToC starting each section, but highliting the current section and optionally showing the subsections of this section only:

% In the preamble!!
\AtBeginSection
{\begin{frame}{ToC}
\tableofcontents[currentsection,hideothersubsections]
\end{frame}}
\begin{document}

And set tocdepth counter to 2 if yo left the frame of point 1!

In some cases (e.g. showing the 3th section) this is not enough because there are 10 subsections. Then these are my suggestions:

a) Do not show any subsections (maintain tocdepth in 1). Still simplicity is power.

b) Redesign the structure, if possible, making fewer subsections.

c) Use multicolumns, e.g.:

% In the preamble!!
\usepackage{multicol}
\AtBeginSection
{\begin{frame}{ToC}
\begin{multicols}{2}
\tableofcontents[currentsection,hideothersubsections]
\end{multicols}
\end{frame}}
\begin{document}

d) Remove the shadow sections, show only the Toc of the actual section with \tableofcontents[sectionstyle=show/hide,subsectionstyle=show/show/hide]:

\documentclass[12pt]{beamer}
\usetheme{Madrid}
\usepackage[american]{babel}
\usepackage[utf8]{inputenx}
\usepackage[T1]{fontenc}
\usepackage{lmodern}

\title{List of Donald Duck universe characters}
\author{Walt Disney}

\begin{document}

\section{Main characters}
\subsection{Donald Duck}
\subsection{Daisy Duck}
\subsection{Huey, Dewey, and Louie}
\subsection{Scrooge McDuck}
\subsection{Ludwig Von Drake}

\section{Relatives}
\subsection{Duck family (Disney)}
\subsection{Clan McDuck}

\section{Duck characters}
\subsection{Umperio Bogarto}
\subsection{Bum Bum Ghigno}
\subsection{Magica De Spell}
\subsection{Evroniani}
\subsection{Flintheart Glomgold}
\subsection{Gloria}
\subsection{Gotrocks}
\subsection{Grand Mogul}
\subsection{Mata Harrier}
\subsection{Brigitta MacBridge}

\begin{frame}[allowframebreaks]{Donald Duck universe characters}
  \tableofcontents[sections={1-2}]
    \framebreak
  \tableofcontents[sections={3}]
\end{frame}

\end{document}

You can specify as option which section to show:

\tableofcontents[sections={1-3}]

So you can manually specify which section to show in each frame:

\begin{frame}{Outline}
  \tableofcontents[sections={1-3}]
\end{frame}
\begin{frame}
   \tableofcontents[sections={4-5}]
\end{frame}