How can i make the text in the first row in the center of cell, and how i can make number "1" in the center of cell as it is indicated with a red rectangle.
\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{cfr-lm} % instead of the "[cyr]{aeguill}"
\usepackage[table]{xcolor}
\usepackage{ragged2e} % new
\usepackage{booktabs, % new
makecell, % new
tabularx} % new
\renewcommand\theadfont{\small\bfseries}
\renewcommand\theadgape{}
\newcolumntype{L}{>{\RaggedRight}X} % redefined
%---------------------------------------------------------------%
\begin{document}
\begin{table}[htb]
\footnotesize
\setlength\tabcolsep{4pt}
\begin{tabularx}{\linewidth}{@{}|
>{\large}c|
>{\hsize=0.25\hsize}L|
>{\hsize=0.25\hsize}L|
>{\hsize=0.50\hsize}L| @{}}
\hline
\thead[l]{RG}
& \thead[l]{Définition}
& \thead[l]{Mapping}
& \thead[l]{Règles de gestion spécifiques} \\
\hline
1 & Portail Date Traitement Courant
& RBP vTBADMRB \_SUIVI\_APPLI.D \_TRAIT
& CASE WHEN
[RBP Presentation View].[Dimension Référentiel Datamart].[L\_DATMR]
= `PORTAIL' THEN [RBP Presentation View].[Fait Suivi Application].[D\_TRAIT] END \\
\hline
\end{tabularx}
\caption{Équipe de travail}
\end{table}
\end{document}


\thead[l]{...}you should write\thead{...}. number one is in the place where you require in one of your previous question. put it in\multirowcell. – Zarko May 27 '18 at 17:33