Is there any settings that limits the number of seeds / peers I can connect to?
I have some downloads that I see have a lot of seeds / peers, but I can’t connect to any of them.
I also have some downloads that always have problem of error tracker response.
Does manual connect help?
How to know if that tracker is still working?
Dont use Manual Connect.
For the tracker error see here
Now since you cant connect to many seeds/peers you probably need to portforward your modem/router.
Read through the speed guides and the settings help in my sig.
Good luck 
My tracker error messages are not listed there.
Error: Can not resolve host address. in Tracker: udp://tracker.bitcomet.net:8080/announce
Error: Tracker Response Error:unregistered torrent in Tracker: http://tk.greedland.net/announce
These are the examples of errors I get. They never connect to any peers and seeds even though I know there are still plenty of seeds and peers.
Also, I only get remote connections, no local connections.
I have done portforwarding and I believe I have done it correctly
"http://tk.greedland.net/announce" was offline for a couple of reasons. It should be up and running again now.
That other address was from someone trying to use the BitComet Tracker and not entering the announce url into the .torrent properly.
So try it again, the greedland tracker should be back up and also make use of DHT & PEX.
What is DHT and PEX?
DHT - Distributed Hash Table = works like a tracker.
PEX - Peer Exchange = other clients exchange IP lists with you.