How many peers should we connect for a single node?
If we connect to many node, will the network be very high and decrease the sync speed?
Or if we only need to connect to 2-3 nodes which may lose connections?
In my experience, 5-10 peers should be enough for ensured connections. It's always a good idea to check the logs periodically to remove unreachable peers and add new peers accordingly. Network connection and disconnection does impact the sync speed.