I propose to use the ltablex package, which brings the functionalities of longtable to tabularx. It will make wider columns (the widest will be approx. 3 cm). However you can see the narrower first column really needs help hyphenating words. You can use a smaller font size, play with the value oftabcolsep and/or change the ratio of column widths. Here are two examples:
\documentclass[a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{geometry}
\usepackage{array, booktabs, siunitx, lscape}
\sisetup{table-number-alignment = center, parse-numbers = false}
\usepackage{ltablex}
\usepackage{ragged2e}
\renewcommand{\tabularxcolumn}[1]{>{\RaggedRight\small\arraybackslash}m{#1}}
\begin{document}
\begin{landscape}
\section{Risico's}
\keepXColumns
\setlength\tabcolsep{3pt}
\begin{tabularx}{\linewidth}{|>{\hsize=0.58\hsize}X | *{7}{>{\hsize=1.06\hsize}X|}}
\caption{Risicoanalyse}\\
\hline
\textbf{Risiconummer} & \textbf{Risico} & \textbf{Kans optreden} & \textbf{Impact} & \textbf{Risicogetal} & \textbf{Preventieve maatregelen} & \textbf{Correctieve maatregelen} \\
\hline
\endfirsthead
\multicolumn{7}{c}%
{\tablename\ \thetable\ -- \textit{Vervolg van de vorige pagina}} \\
\hline
\textbf{Risiconummer} & \textbf{Risico} & \textbf{Kans optreden} & \textbf{Impact} & \textbf{Risicogetal} & \textbf{Preventieve maatregelen} & \textbf{correctieve maatregelen} \\
\hline
\endhead
\hline \multicolumn{7}{r}{\textit{Vervolg op de volgende pagina}} \\
\endfoot
\hline
\endlastfoot
asfdfasfas\-dfdf & asdfasfdasfdsa fadsfaafdssdfdsfasdf & asdfasdfa sfsadfasf & adfafas dfsafsdf & adfasfas dfasdfsa & 6 & 7
\end{tabularx}
\begin{tabularx}{\linewidth}{|>{\hsize=0.86\hsize}X | *{7}{>{\hsize=1.02\hsize}X|}}
\caption{Risicoanalyse}\\
\hline
\textbf{Risiconummer} & \textbf{Risico} & \textbf{Kans optreden} & \textbf{Impact} & \textbf{Risicogetal} & \textbf{Preventieve maatregelen} & \textbf{Correctieve maatregelen} \\
\hline
\endfirsthead
\multicolumn{7}{c}%
{\tablename\ \thetable\ -- \textit{Vervolg van de vorige pagina}} \\
\hline
\textbf{Risiconummer} & \textbf{Risico} & \textbf{Kans optreden} & \textbf{Impact} & \textbf{Risicogetal} & \textbf{Preventieve maatregelen} & \textbf{correctieve maatregelen} \\
\hline
\endhead
\hline \multicolumn{7}{r}{\textit{Vervolg op de volgende pagina}} \\
\endfoot
\hline
\endlastfoot
asfdfasfas\-dfdf & asdfasfdasfdsa fadsfaafdssdfdsfasdf & asdfasdfa sfsadfasf & adfafas dfsafsdf & adfasfas dfasdfsa & 6 & 7
\end{tabularx}
\end{landscape}
\end{document}
