dofollow signature
 

I use nofollow throughout the forum. I want to allow dofollow for the signature with minimum 20 post.

Is there any Plugin ?

I don't know any plugin for that but you could create 2 separate user groups and then edit the way links are parsed for both groups. I know there is a possibility to disable posting links at all until a user gets certain number of posts, perhaps tweaking this option would do the thing.