How can I use a 'for' loop to display 50 pictures on a page?

Question

I want to show 50 pictures with numbers from 1 to 50 beneath on one page (or only a few pages). So basically a list of images with a number to identify them.

I tried:

\documentclass{article}
\usepackage{graphicx}
\usepackage{pgffor}

\begin{document}
\foreach \x in {1,...,\numimages}{
    \begin{figure}
            \includegraphics[width=.2\columnwidht]{./img/img_\x}
            \caption{\x}
    \end{figure}
\fi
\end{document}

The images are named "img_1" to "img_50". They are in the folder img.

Problems

1. The name is "Figure 1: 1" and it should only be "1"
2. The images start as a new line and do not appear side by side

How it looks like now:

How it should look like (just the arrangement and the caption, not the paper):

Answer 1

The only advantages of using the figure environment is that it will float to a good position in your document and the ability to add a proper caption with "Figure 42: ..." - two things which seem undesired in your case, so don't use a figure environment.

Without it, you can place the images besides each other like normal letters in a text:

\documentclass{article}
\usepackage{graphicx}
\usepackage{pgffor}

\begin{document}

{%
\noindent%
\raggedright%
\foreach \x in {1,...,50}{%
  \parbox{.18\textwidth}{\centering\includegraphics[width=\linewidth]{example-image-duck}\par \x}\space%
}%

\end{document}

Answer 2

An abstraction seems the best strategy. I define the \multipleimages command with the syntax

\multipleimages
  {<file name pattern>}
  {<number of images per line>}[<correction>]
  {<total number>}

In your case the call should be

\multipleimages{./img/img_#1}{5}{50}

or maybe

\multipleimages{./img/img_#1}{5}[0.8]{50}

The optional correction factor can be determined by trying to typeset reducing it until the float can find place in a page. In the example below I get

LaTeX Warning: Float too large for page by 11.14352pt on input line 45.

when the correction factor is set to 0.8, but no such warning with 0.79. The right factor depends on the vertical size of the images, which are supposed to have the same height.

The second example shows how to change the number of images per line.

There is no #1 in the first argument in examples 1 and 2 because I didn't want to create 50 different names. So, for the third example, I made four copies of example-image.pdf named img1.pdf, img2.pdf, img3.pdf and img4.pdf in order to show that the first argument correctly interprets the given pattern.

\documentclass{article}
\usepackage{graphicx}

\ExplSyntaxOn

\NewDocumentCommand{\multipleimages}{mmO{1}m}
 {% #1 = pattern for the image names
  % #2 = number of images per line
  % #3 = reduction factor (default 1)
  % #4 = total number of images
  \curious_multimg:nnnn { #1 } { #2 } { #3 } { #4 }
 }

\int_new:N \l__curious_multimg_idx_int

\cs_new_protected:Nn \curious_multimg:nnnn
 {
  \cs_set:Nn \__curious_multimg_name:n { #1 }
  \int_zero:N \l__curious_multimg_idx_int
  \int_step_inline:nn { #4 }
   {
    \group_begin:
    \renewcommand{\arraystretch}{0}
    \begin{tabular}[t]{@{}c@{}}
    \includegraphics[width=\fpeval{(#3)/(#2)}\columnwidth]{\__curious_multimg_name:n { ##1 }}
    \\
    \footnotesize\strut ##1
    \end{tabular}
    \group_end:
    \int_compare:nT{\int_mod:nn{##1}{#2}==0}{\par\vspace{0.3ex}}
   }
 }

\ExplSyntaxOff

\begin{document}

\begin{figure}[p]
\centering

\multipleimages{example-image}{5}[0.79]{50}

\caption{Fifty images}

\end{figure}

\begin{figure}[p]
\centering

\multipleimages{example-image}{7}{50}

\caption{Fifty images}

\end{figure}

\begin{figure}[p]
\centering

\multipleimages{img#1}{2}{4}

\caption{Four images}

\end{figure}

\end{document}