This is not a rule but a timely advice to newbie who wants to keep their posting reputation in good standard.
One meaningful post is better than 15 to 20 post that are low in quality just to fulfill signature requirements.
Most campaign managers accept particpants from Full Member rank and above, they don't usually accept Member or Jr Member rank. The benefits of signature campaigns outweighs its disadvantages and it incentivizes quality posts and better users, for example, if you constantly make low quality posts, it reduces your chances of joining a campaign and if you are already in one, it increases your chances of getting kicked out.
That being said, if you know you'd not be able to meet the posting requirements of a campaign without spamming, it is better not to apply for such campaigns and wait for one that suits you.