documentclass{article}
usepackage{subcaption}
usepackage{graphicx}

begin{document}

begin{figure}
centering
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testa}
end{subfigure}%
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testb}
end{subfigure}%
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testc}
end{subfigure}%
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testd}
end{subfigure}parmedskip
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:teste}
end{subfigure}%
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testf}
end{subfigure}%
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testg}
end{subfigure}%
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testh}
end{subfigure}parmedskip
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testi}
end{subfigure}%
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testj}
end{subfigure}%
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testk}
end{subfigure}%
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testl}
end{subfigure}parmedskip
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testm}
end{subfigure}%
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testn}
end{subfigure}%
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testo}
end{subfigure}%
begin{subfigure}{.25linewidth}
centering
includegraphics[width=.8linewidth]{example-image-a}
caption{A subfigure}
label{sfig:testp}
end{subfigure}
caption{Sixteen subfigures}
label{fig:test}
end{figure}

end{document}