Do I need to meet a certain number of character on a post to be counted as an activity? Like word count or minimum character count? Or it doesn't matter as long as my post has sense?
No. None of any of this matters for activity. One post is one post regardless of whether it's 1 character or an entire novel. However, if you make poor posts you run the risk of being banned so take that into consideration if you plan on just making short posts merely to collect activity.