Read more tag..

You may want to try to use this plugin:
https://guff.szub.net/2005/02/26/the-excerpt-reloaded/

If you want valid XHTML at all times, best not to attempt it – catching and closing all tags is very hard work with the functions PHP has available…

That plugin doesn’t work good for me. First, if i set the excerpt length to, for example: 20 (words) it still shows all the words.
And i always use pictures in my entries, and with this plugin they disappear.

There’s the Evermore plugin, plus Bennett lists a number of alternatives:
https://www.thunderguy.com/semicolon/wordpress/evermore-wordpress-plugin/

Hmm, the Evermore plugin works with paragraphs, and i want it with characters. The others also doesn’t work how i want.

First, if i set the excerpt length to, for example: 20 (words) it still shows all the words.
And i always use pictures in my entries, and with this plugin they disappear.

Are we talking about the_excerpt_reloaded? Just enabling it isn’t enough — you have to actually use the template tag in your theme. Be sure to read the documentation at the author’s page.