Number of ads per Usergroup
 

Hello
I didn't understand how that works. For example my Registered users is set
Max Ads Limit to 0
Free Ads set to 3.
So, I've thought that the users will be limited for 3 free ads, but really that's not work, registrered users can post more than 3 ads on my classifieds

Free ads is the number of free ads they are allowed before paying for an ad. That means that they can place 3 ads for free if they are posting and you have setup payment.

Max ads means explicitly that number of ads overall they can post.

Free ads is useless unless your making registered users pay to post and have properly setup payment options ;)