In the beginning, I could save each animation in a single png file.
Now it saves to me as dozens of files.
Where is the problem?
Hi and welcome here 
Simply check “Render current frame only” in Render Setting
Thank you very much, problem solved. 
Could you please help me again? Because I don’t want to start a new topic.
I am trying to create a simple animation of drawing or outlining contours.
Can I animate (point by point) a previously created Advanced Outline on a graphics tablet?
To make it look like a drawing.
Use this plugin to do your animation.
Thank you very much. 