While I am connected to a node, listing to a repeater. Can another node connect to me? Which one has priority ? Should I disconnect from the first node, before responding to the second node?
Any node that you have not blacklisted can connect to your node if it is public. Unless there's malicious or connection degrading behavior, I don't do anything other than carry on as normal. All incoming connections not blacklisted have equal priority.
To add to this:
Some large hubs really don't like it if other people show up on a node that is connected to them for various reasons. If it's just a few ad-hoc nodes talking to each other where this sort of thing doesn't really matter, then I wouldn't worry about it too much.
True. I have had to assure some local nets (when I am testing ASL) that, no, I am not bringing 50 friends to your net, I am connected in Monitor mode. 
Sorry about these questions. This is all brand new to me.
I just don't want to bring other nodes with me, when I connect to a friends repeater node. The second node that connected to me, as I'm listening to the repeater node, will that second node also be connected to the repeater node?
Thank You
Yes it will also be connected to the repeater. Your node will act as a bridge and all traffic on the repeater will be heard on the second node. Likewise, any traffic on the second node will be heard on the repeater. If you are using ASL 3, have a look at the Node Map on the Allmon3 tool. This is the first icon in the top right of the screen. If you are using Supermon it is the Bubble button. The resulting chart will show all nodes connected to each other and you will likely see examples of bridges. A big problem occurs when a node is connected to a large hub, e.g. East Coast Reflector, and then connects to another large hub such as HubNet. So then you have hundreds of nodes connected to each other which will put a strain on the system. You can put in measures to reduce the risk of this happening. By using scripts to handle connections you can perform a disconnect first before making a connection to another node. You can also use some of the advanced cop commands to disable incoming connections/disable linking etc. when connected to a large hub.
Yes, it is not a peer-peer configuration, but a multi peer configuration.
As already stated, you can specifically allow only or deny specific stations.
And the limit to how many connections is really about your available resources.
But as I remember it, there was a parameter that could be set for max connections, but it escapes me at this moment. It was only used to keep systems from crashing when you were near known limits as far as I know.
Ya, it's MAX_STAT_LINKS in the source. Currently it's defined as 256.
Thank you for your help