BTW I'm still trying to find out how many nodes there are (if you know). Isn't there a count somewhere?
What? It is supposed to slow down miners, that is exactly the point of the difficulty. While you may be a greedy bastard wanting to mine at a lower difficulty than everyone else, the rest of us know that would break bitcoin, in that case whatever you managed to mine would be worthless, so enjoy.
There is no count on the nodes, they come and go as they please. A node is a lot like a bittorrent client, some are well established and never move. Others only run when necessary and don't do much else. Why do you want a count anyway? What would that tell you?
You may have seen the messages on blockchain.info, but that is not on the bitcoin blockchain at all. I think there may have been a way to add text to a transaction, but I don't think it was ever used/enabled. It still is possible to send "messages" in the blockchain, but that's more complicated than I can explain.
I'm a newbie here, so pls. correct me if my understanding is wrong.
You are exactly right. There's nothing stopping a node from mining blocks with no transactions at all, as well.
What about signing message? Is that same thing or doesnt add to blockchain?
No, signing a message just uses the public and private key pair from a bitcoin transaction to verify ownership of the private key, without revealing it. The blockchain is used as reference only, no information is added.