I mostly reply to threads. If it's a newer forum or one that's revamping, I may post a few threads to help kickstart some conversations.
On my forum I start most of the threads.
I will reply to threads that interest me but I will also reply to those that have 0 replies to hopefully get a discussion going. I've found that has worked in several forums I'm in.
How many posts? On mine it can vary. On others, I try to do several.
Sounds like you're a great person to have on a forum then! So thoughtful.