It's not based on number of posts, You can have a sig when you become an Established Member, It's all explained here: https://www.digitalpoint.com/help/established
If you are comeing for forums to only for post your signature then it's also spam, But if you post some great stuff and share your experience with other members it's good.
Forget on how many post until you reach the benefit of having signature but contribute on this reputable forum and we can have a healthy discussions here. Cheers!
surly we can have a healthy environment to talk ans share on related matters but some members don't event think about.