Change the font size in a table or settings of the table

I see mainly tow suggestions

  1. Allow the tcolorbox to trample on the margins by enlarging it's width.
  2. Reduce the font size

%

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{geometry}
\usepackage{tcolorbox}
\usepackage{tabularx}
\usepackage{array}
\usepackage{colortbl}
%\DeclareFloatFont{tiny}{\tiny}
\tcbuselibrary{skins}

%\newcolumntype{Y}{>{\raggedleft\arraybackslash}X}

\tcbset{tab2/.style={enhanced,fonttitle=\bfseries,fontupper=\normalsize\sffamily,
colback=yellow!10!white,colframe=red!50!black,colbacktitle=Salmon!40!white,
coltitle=black,center title}}

\begin{document}
\begin{table}[ht]
    \centering
\begin{tcolorbox}[tab2,
tabularx={X|*{14}{|c}},
title=Bestandteile von T/P92,boxrule=0.5pt,
,grow to left by=0.5cm,
grow to right by=0.5cm]
 & C & Mn & P & S & Si & Cr & W & Mo & V & Cb(Nb) & N & B & Al & Ni      \\\hline\hline
min~\%   & 0.07& 0.3 &  Ns &  Ns & Ns& 8.5& 1.5 & 0.3 &  0.15 & 0.04& 0.03 &  0.001 &  Ns & Ns \\
max~\% & 0.13& 0.02 &  0.01 &  0.5 & 9.5& 2& 0.6 &  0.6 &  0.25 & 0.09& 0.07 &  0.006 &  0.04 & 0.4\\\hline\hline
\end{tcolorbox}
  \caption{Caption}
    \label{tab:my_label}
\end{table}

\begin{table}[ht]
    \centering
\begin{tcolorbox}[tab2,
tabularx={X|*{14}{|c}},
title=Bestandteile von T/P92,boxrule=0.5pt,
fontupper=\small\sffamily, 
% or fontupper=\footnotesize \sffamily,
]
 & C & Mn & P & S & Si & Cr & W & Mo & V & Cb(Nb) & N & B & Al & Ni      \\\hline\hline
min~\%   & 0.07& 0.3 &  Ns &  Ns & Ns& 8.5& 1.5 & 0.3 &  0.15 & 0.04& 0.03 &  0.001 &  Ns & Ns \\
max~\% & 0.13& 0.02 &  0.01 &  0.5 & 9.5& 2& 0.6 &  0.6 &  0.25 & 0.09& 0.07 &  0.006 &  0.04 & 0.4\\\hline\hline
\end{tcolorbox}
   \caption{Caption}
    \label{tab:my_label2}
\end{table}
\end{document}

enter image description here


Building upon Hafid Boukhoulda's solution, I have slightly decreased the tabcolsep. With this, the table can fit into the textwidth without the need to decrease the font size and without overfull boxes. Please note that I have also used the geometry package to get a wider textblock.

enter image description here

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{geometry}
\usepackage{tcolorbox}
\usepackage{tabularx}
\usepackage{array}
\usepackage{colortbl}

\tcbuselibrary{skins}


\tcbset{tab2/.style={enhanced,fonttitle=\bfseries,fontupper=\normalsize\sffamily,
colback=yellow!10!white,colframe=red!50!black,colbacktitle=Salmon!40!white,
coltitle=black,center title}}

\begin{document}
\begin{table}[ht]
    \centering \setlength{\tabcolsep}{5.25pt}
\begin{tcolorbox}[tab2,
tabularx={X|*{14}{|c}},
title=Bestandteile von T/P92,boxrule=0.5pt,
%fontupper=\small\sffamily, 
]
 & C & Mn & P & S & Si & Cr & W & Mo & V & Cb(Nb) & N & B & Al & Ni      \\\hline\hline
min~\%   & 0.07& 0.3 &  Ns &  Ns & Ns& 8.5& 1.5 & 0.3 &  0.15 & 0.04& 0.03 &  0.001 &  Ns & Ns \\
max~\% & 0.13& 0.02 &  0.01 &  0.5 & 9.5& 2& 0.6 &  0.6 &  0.25 & 0.09& 0.07 &  0.006 &  0.04 & 0.4\\\hline\hline
\end{tcolorbox}
   \caption{Caption}
    \label{tab:my_label2}
\end{table}
\end{document}

If you don't want to use the geometry package you might be interested in one of the following two suggestions:

enter image description here

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{tcolorbox}
\usepackage{tabularx}
\usepackage{array}
\usepackage{colortbl}

\tcbuselibrary{skins}


\tcbset{tab2/.style={enhanced,fonttitle=\bfseries,fontupper=\normalsize\sffamily,
colback=yellow!10!white,colframe=red!50!black,colbacktitle=Salmon!40!white,
coltitle=black,center title}}

\begin{document}

\begin{table}[ht]
    \centering \setlength{\tabcolsep}{2.99pt}
\begin{tcolorbox}[tab2,
tabularx={X|*{14}{|c}},
title=Bestandteile von T/P92,boxrule=0.5pt,
fontupper=\small\sffamily, 
]
 & C & Mn & P & S & Si & Cr & W & Mo & V & Cb(Nb) & N & B & Al & Ni      \\\hline\hline
min~\%   & 0.07& 0.3 &  Ns &  Ns & Ns& 8.5& 1.5 & 0.3 &  0.15 & 0.04& 0.03 &  0.001 &  Ns & Ns \\
max~\% & 0.13& 0.02 &  0.01 &  0.5 & 9.5& 2& 0.6 &  0.6 &  0.25 & 0.09& 0.07 &  0.006 &  0.04 & 0.4\\\hline\hline
\end{tcolorbox}
   \caption{Caption}
    \label{tab:my_label2}
\end{table}

\begin{table}[ht]
    \centering \setlength{\tabcolsep}{3.65pt}
\begin{tcolorbox}[tab2,
tabularx={X|*{14}{|c}},
title=Bestandteile von T/P92,boxrule=0.5pt,
fontupper=\footnotesize\sffamily, 
]
 & C & Mn & P & S & Si & Cr & W & Mo & V & Cb(Nb) & N & B & Al & Ni      \\\hline\hline
min~\%   & 0.07& 0.3 &  Ns &  Ns & Ns& 8.5& 1.5 & 0.3 &  0.15 & 0.04& 0.03 &  0.001 &  Ns & Ns \\
max~\% & 0.13& 0.02 &  0.01 &  0.5 & 9.5& 2& 0.6 &  0.6 &  0.25 & 0.09& 0.07 &  0.006 &  0.04 & 0.4\\\hline\hline
\end{tcolorbox}
   \caption{Caption}
    \label{tab:my_label2}
\end{table}
\end{document}

I propose using tabular* together with siunitx.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[usenames,dvipsnames,table]{xcolor}
\usepackage{tabularx}
\usepackage{array}
\usepackage{siunitx,booktabs}
\usepackage{colortbl} % better passing table to xcolor
\usepackage{tcolorbox}
\tcbuselibrary{skins}

\makeatletter
\tcbset{
  tab2/.style={
    enhanced,
    fonttitle=\bfseries,
    fontupper=\normalsize\sffamily,
    colback=yellow!10!white,
    colframe=red!50!black,
    colbacktitle=Salmon!40!white,
    coltitle=black,
    center title
  },
  tabular*/.style={%
    boxsep=\z@,top=\z@,bottom=\z@,leftupper=\z@,rightupper=\z@,
    toptitle=1mm,bottomtitle=1mm,boxrule=0.5mm,
    before upper*={\arrayrulecolor{tcbcolframe}\def\arraystretch{1.1}%
      \expandafter\tcb@hack@currenvir\csname tabular*\endcsname{\linewidth}{#1}},
    after upper*=\csname endtabular*\endcsname\arrayrulecolor{black}},
}
\makeatother

\newcolumntype{T}[1]{S[table-format=#1]}

\begin{document}

\begin{table}[ht]
\centering

\setlength{\tabcolsep}{0pt}
\sisetup{detect-all}
\begin{tcolorbox}[
  tab2,
  tabular*={
    @{\extracolsep{\fill}\hspace{2pt}}
    l *{3}{T{1.2}} *{5}{T{1.1}} *{3}{T{1.2}} T{1.3} T{1.2} T{1.1}
    @{\hspace{2pt}}
  },
  title=Bestandteile von T/P92,
  boxrule=0.5pt
]
 & {C} & {Mn} & {P} & {S} & {Si} & {Cr} & {W} & {Mo} & {V} & {Cb(Nb)} & {N} & {B} & {Al} & {Ni}
\\
\midrule
min\% & 0.07& 0.3  & {Ns} & {Ns} & {Ns} & 8.5 & 1.5 & 0.3 & 0.15 & 0.04 & 0.03 & 0.001 & {Ns} & {Ns}
\\
max\% & 0.13& 0.02 & 0.01 & 0.5  & 9.5  & 2   & 0.6 & 0.6 & 0.25 & 0.09 & 0.07 & 0.006 & 0.04 & 0.4
\\
\end{tcolorbox}

\caption{Caption}
\label{tab:my_label}

\end{table}

\end{document}

The tabular* style is not available with the standard tcolorbox, but it would be an interesting addition.

enter image description here

After looking at the typeset table, it will be immediately apparent whether reducing the font size with \small or \footnotesize is necessary.