Using the subfig package, I would personally suggest the following example:
\documentclass[a4paper, 10pt]{article}
\usepackage{graphicx}
\usepackage{subfig}
\begin{document}
\begin{figure}[htbp]
\centering
\subfloat[1]{{\includegraphics[width=0.22\textwidth]{example-image-a}}}
\hfill
\subfloat[2]{{\includegraphics[width=0.22\textwidth]{example-image-a}}}
\hfill
\subfloat[3]{{\includegraphics[width=0.22\textwidth]{example-image-a}}}
\hfill
\subfloat[4]{{\includegraphics[width=0.22\textwidth]{example-image-a}}}
\subfloat[1]{{\includegraphics[width=0.22\textwidth]{example-image-a}}}
\hfill
\subfloat[2]{{\includegraphics[width=0.22\textwidth]{example-image-a}}}
\hfill
\subfloat[3]{{\includegraphics[width=0.22\textwidth]{example-image-a}}}
\hfill
\subfloat[4]{{\includegraphics[width=0.22\textwidth]{example-image-a}}}
\caption{graphs}%
\end{figure}
\end{document}
I have specified the width of every image to be 22% of the textwidth. That way the images still fit even if the margins are changed. \hfill between neighbouring images ensures, that the images are horizontally evenly spread over the whole textwidth.
update
As requested in the comments, this update includes a MWE showing the images in 2 columns with 4 rows each (instead of 4 columns with 2 rows).
\documentclass[a4paper, 10pt]{article}
\usepackage{graphicx}
\usepackage{subfig}
\begin{document}
\begin{figure}[htbp]
\centering
\subfloat[1]{{\includegraphics[width=0.45\textwidth]{example-image-a}}}
\hfill
\subfloat[2]{{\includegraphics[width=0.45\textwidth]{example-image-a}}}
\subfloat[3]{{\includegraphics[width=0.45\textwidth]{example-image-a}}}
\hfill
\subfloat[4]{{\includegraphics[width=0.45\textwidth]{example-image-a}}}
\subfloat[1]{{\includegraphics[width=0.45\textwidth]{example-image-a}}}
\hfill
\subfloat[2]{{\includegraphics[width=0.45\textwidth]{example-image-a}}}
\subfloat[3]{{\includegraphics[width=0.45\textwidth]{example-image-a}}}
\hfill
\subfloat[4]{{\includegraphics[width=0.45\textwidth]{example-image-a}}}
\caption{graphs}%
\end{figure}
\end{document}
\\which indicates a linebreak after the second and third image. – leandriis Oct 06 '17 at 09:15