\documentclass{article}
\usepackage{xcolor}
\usepackage{ltxkeys}
\makeatletter
\def\somemethod#1#2{%
  \begingroup
  \color{#1}%
  \ifltxkeys@novalue
    \ifnum\ltxkeys@listcount=\@ne
      \endgraf
      Using default value of `\ltxkeys@tkey':
    \fi
  \else
    \ifnum\ltxkeys@listcount=\@ne
      \endgraf
      Using user value of `\ltxkeys@tkey':
    \fi
  \fi
  \endgroup
  \endgraf
  Processing item no. \ltxkeys@listcount: #2%
}
% Use comma (,), the default list separator, as the list separator
% for keya and keyb:
\ltxkeys@listkey[KV]{fam}{keya,keyb}[default1,default2]{%
  \somemethod{blue}{#1}%
  % Process only four (4) items of the list/value for keya and keyb:
  \ifnum\ltxkeys@listcount=4\relax
    \stopprocessinglistkey
  \fi
}

% Use semicolon (;) as the list separator for keyc and keyd:
\ltxkeys@listkey[KV]{fam}{keyc,keyd}[default1;default2]{%
  \listsep{;}\somemethod{red}{#1}%
}

\makeatother

\begin{document}
\parindent0pt
% No values for keya and keyb; use their default values:
\ltxkeysusekeys[KV]{fam}{keya,keyb}

\par\bigskip
\ltxkeysusekeys[KV]{fam}{%
  keya={item1A,item1B,item1C,item1D,item1E,item1F},
  keyb={item2A,item2B,item2C,item2D,item2E,item2F}
}

\par\bigskip
% Set keyc and keyd with their default values:
\ltxkeysusekeys[KV]{fam}{keyc,keyd}

\par\bigskip
\ltxkeysusekeys[KV]{fam}{%
  keyc={item1A;item1B;item1C;item1D},
  keyd={item2A;item2B;item2C;item2D}
}
\end{document}

